Hollow Rattle/Light Knocking Under Car

[Windy_3V]

So I was driving home from work today and noticed an unsettling noise coming from the underside of the car. I had the music cranked and I was able to just catch it through the music. I stopped immediately and got out to check over the car but didn’t find anything and proceeded to drive home since I was only a mile away.

Best way I can describe it is a hollow rattling or knocking. Only happens when the car is in gear accelerating or decelerating. It’s not necessarily rhythmic but the noise does increase under higher load and gets faster at higher speeds. If I put it in neutral while rolling it goes away. It’s not present while idling at a stop or revving the engine at any rpm either.

I doubt it’s the motor but I could be wrong. I just changed my oil a few hundred miles ago and checked it when I pulled over and it’s fine. I suspect it’s transmission or drivetrain related since they’re pretty tired and most likely on the way out. Still has the stock 5-speed manual with 206k miles with a McLeod RXT twin disc clutch and MGW short throw which have been in the car since I bought it about 3 years ago (Yes I know I should probably replace my clutch lol)

Just wanted to see if anyone had a similar issue and/or can point me in the right direction of a possible issue. Haven’t really had a chance to get a good look at everything since it was already dark out so I’ll have to wait till tomorrow.

Might be my final kick in the butt to motivate me to do that TR-6060 swap I’ve been wanting to do

 

[Samos3]

Not necessarily Mustang specific, but I find rpm related 'hollow' sounds to be heat shields (like on a catalytic converter/muffler) or the driveshaft is resonating (big hollow tube).

 

[Windy_3V]

I’d have to check tomorrow to see if there’s anything loose under the car. My car is catless so I can rule that out. Driveshaft was one of my first thoughts but it would still be spinning in neutral which makes me believe it’s not that either.

 

[Windy_3V]

So I was driving home from work today and noticed an unsettling noise coming from the underside of the car. I had the music cranked and I was able to just catch it through the music. I stopped immediately and got out to check over the car but didn’t find anything and proceeded to drive home since I was only a mile away.

Best way I can describe it is a hollow rattling or knocking. Only happens when the car is in gear accelerating or decelerating. It’s not necessarily rhythmic but the noise does increase under higher load and gets faster at higher speeds. If I put it in neutral while rolling it goes away. It’s not present while idling at a stop or revving the engine at any rpm either.

I doubt it’s the motor but I could be wrong. I just changed my oil a few hundred miles ago and checked it when I pulled over and it’s fine. I suspect it’s transmission or drivetrain related since they’re pretty tired and most likely on the way out. Still has the stock 5-speed manual with 206k miles with a McLeod RXT twin disc clutch and MGW short throw which have been in the car since I bought it about 3 years ago (Yes I know I should probably replace my clutch lol)

Just wanted to see if anyone had a similar issue and/or can point me in the right direction of a possible issue. Haven’t really had a chance to get a good look at everything since it was already dark out so I’ll have to wait till tomorrow.

Might be my final kick in the butt to motivate me to do that TR-6060 swap I’ve been wanting to do

Update as of 1/29/25

I did a thorough check of the car and still nothing visibly wrong with the car. However, after jacking up the rear wheels and spinning them by hand, i’m able to replicate the noise. I had my doubts but now I’m leaning more towards driveshaft. Could be rear diff or axles as well. Imo I believe the 2-piece driveshaft is the weak link in the drivetrain.

With that being said I’m thinking about just overhauling the drivetrain since everything is fairly worn out. I’d like to do a T56/TR6060 swap, McLeod Twin Disc, 8-bolt lightweight flywheel, one piece driveshaft, and 3.73 gears that I’ve had lying around. If anyone has any advice, opinions, or suggestions feel free to drop a reply

 

[MrBhp]

That 8 bolt flywheel will need to be bolted to an 8 bolt crankshaft. I don't remember your build. Did you upgrade to forged rotating assembly?

 

[DieHarder]

I’d have to check tomorrow to see if there’s anything loose under the car. My car is catless so I can rule that out. Driveshaft was one of my first thoughts but it would still be spinning in neutral which makes me believe it’s not that either.

Actually, the driveshaft doesn't spin in neutral. Not until engaged by the clutch.

 

[GriffX]

Actually, the driveshaft doesn't spin in neutral. Not until engaged by the clutch.

If rolling in neutral it spins only the load changes from forward to backward, maybe a U-joint or the slip joint (?)

 

[Windy_3V]

Posting an update in case people find this thread and are curious. I ended up finding a really good deal on an entire rear end. Completely refreshed 8.8 all new bearings and seals, Yukon 31-spline axles, Diff locker, beefy Yukon 3.73 gears etc. for $400. After inspecting my old rear end, it turns out my pinion shaft seal was blown and causing the pinion to have a crazy amount of play making that noise I was hearing. Now that I know the problem, I suspect it was worn for a long time (40,000+ miles) and it finally gave out based on noises and stuff I was hearing prior to the actual failure. All is good with the new rear end. Put about 1,500 miles on it so far and it’s been great. Gained some cool race car noises too thanks to the beefy components and the lockers are a nice plus. Thanks for all your comments and suggestions!!

 

[JoeB]

I put a Yukon locker &373 gear in my 2006. Gear is after market, & noisy as hell between 45 & 80 MPH! Wish I had used the stock 331 gear, on the Yukon locker. The 373 also makes my 4.0 eat a lot more gas.

 

[brasil]

same here... my 2008 Bullitt had the 3.73 gears. they made some noise from the first day on. Also the "play " between ring and pinion was bigger than the the play that my 3.31 now has. Interesting is also, that the Bullitt had driven only 20 k mls... and the 2010 that I own now...has 70 k mls on the odometer.
Milage is better with the 3.31... on the Autobahn. Around town.. same numbers..

 

[lenko]

The hangar arm on my left rear muffler (tailpipe) would touch the back bumper when the pipes heated up so I ground a bit off the arm and haven't heard it since. That was probably 2006 or 2007 shortly after I bought the car new. Exhaust pipes were expanding (getting longer)when they heated up. Could this be your problem?

 

[metrolink]

The hangar arm on my left rear muffler (tailpipe) would touch the back bumper when the pipes heated up so I ground a bit off the arm and haven't heard it since. That was probably 2006 or 2007 shortly after I bought the car new. Exhaust pipes were expanding (getting longer)when they heated up. Could this be your problem?

This ^ and also I had somewhat of a similar issue on mine. I was getting rattling sounds at certain rpms and it turned out one of my exhaust hangers had snapped, so the exhaust was vibrating closely against metal. Also, do you notice the noise right after a cold start, or only once you're driving?

 

[lenko]

Only when driving as the exhaust system heated up and elongated.

 

[odie1]

I had a rattle in my 06 gt convertible, and couldn't find anything loose underneath. Someone here suggested checking the spare tire for hold down tightness. The wing nut had managed to back off enough, or was never properly retightened at some point. I got about 6 to 8 spins on it, and poof, rattle gone. Sometimes its the simple stuff.

 

[rocknrod]

Similar issues here as well. My spare tire lug wrench doesnt stay in one spot. I can here it sometimes. So I zip tied it to the spare. No noises anymore.