Knocking from the rear... completely lost.

history:
2005 gt, 5 speed, 127xxx miles, stock suspension, stock exhaust.

symptoms:
knocking from driver side rear on deceleration when lifting completely off the throttle, goes away when accelerating or maintaining speed as long as the throttle is on. Sounds like a loose shock or lugs nuts (for example). Its not rhythmic (like a rock in the tire) but does "rattle" faster at higher speeds. occurs when clutch is depressed and engaged. coasting in neutral and reving the engine does not change it. applying the breaks does not make it go away. does not occur 100% if the time but, occurs more often than not. turning does not produce a sound.

checked:
suspension bushing loaded and unloaded look good, no play. changed sway bar bushing, even driven without sway-bar. drive shaft bolts appear to be tight. no roar when spinning the wheels by hand. no marks on the exhaust, plenty of clearance. brake calipers and lugs are tight. there is a little play pull the wheels in and out and lifting the wheels up, it is the same on both sides though.

side note:
put in gear on jack stands ran it up and let it decel in gear and in neutral. there was no knocking...
really not looking forward to taking it to the dealership. i know they will throw parts at it til its fixed and i will have to pay for them all needed or not...

anyone have an idea as to the cause of this? thanks in advance!

First thing I thought of is a Ujoint starting to go

checked the U-joint at the transmission looks good, no play. I did notice there is a little play between the rear section of the drive shaft and the front, just enough to make a sound when i twist it back and forth. should I be able to twist the driveshaft any with the ebrake locked?

Yes, the play is the gear backlash.

Ujoimts usually get tight and start to make noise before they get loose. Look at the seals (rubber Orings) on the Ujoints. If you see some rust coming from one or more, that Ujoint is on the way out.

okay thanks I will check.

checked the U joint. the rubber seals are intact, a little grease. there is surface rust on the caps and clips, but not at the inner portion of the joint. pushed, pulled, and twisted the drive shaft the U joint didn't move. but, I drove it, and it and noticed will knock when I swerve left, nothing to the right. its seems as if it starts when the weight is shifted off the rear. sounds like its coming from the left rear corner,

Did you check the muffler hangar mounting bolts? I had a slight knocking and one of the hangar bolts that screws into the frame rails had come a little lose which allowed the hangar bracket to rattle/knock against the frame rail.

yep. just put a wrench on them to double check. put weather stripping between the fuel neck and wheelwell, found a bracket loose holding the fuel lines in place, its fixed. took it to the dealer this morning for a ride-along. mechanic thinks its something loose, not in the drive train. when i got home put it on jack stands under the axle, ran through the gears, no noise. only occurs when driving. odd that it varies with speed i can stop it under acceleration, yet there is no roar, whining, vibration, and its not consistant.

My 08 gt did the same thing with knocking when off the gas and coasting. My bearing cap inside rear end on passenger side was chewed up for some ungodly reason. Had to replace whole rear end. Hope you don’t have that kind of luck. Might want to check your limited slip differential.


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thanks. I think i'm going to go ahead and replace the spider gears and axle bearings since ill have to pull the rear end apart. if that doesn't fix it i'll give it to the dealer.

One time I had an smal rattle can (spray paint) that had fallen down into the small poecket in the fenders

update... 4 mechanics, transmission shop, ford dealer still not fixed!

it now has new shocks, sway bar bushings, side gears, spider gears, carrier bearings, pinion bearings, brake caliper, rotor, brake pads, Moser axles, and lower control arms.

used a chassis ear on the Left lower control arm, diff housing, drive shaft carrier, and transmission. knocking/rattling was loudest from the control arm, very little on the diff, nothing on the transmission or driveshaft carrier.

HELP! please and thank you!

Based on what you have checked and repaired, I know this is unlikely but . . . when I bought mine (2nd owner), I had an consistent rattle that almost sounded like someone smacking the back window with a wrench.

I suspected, and was right, something had broken loose in the aftermarket mufflers. When I changed them out, the rattle was gone.

ill give it a shot thanks.

how do you post a video on here?

I use the old fashion method . . . upload to a free photobucket account and then cut/paste the link into this thread.

you mean literally any other site

fuck photobucket and their hotlinking hostage making

figured it out, maybe... Seems like two issues.

One- the caliper is not pressing the pads completely, have to pump it maybe 2-3 times to fully lock.
two- with the car on the ground I can pull on the top of the rear wheel the rotor will hit the pads and knock.

Should i be able to slide the axle with the weight of the car on it? the passenger side does not move but the brakes don't have any slack either.

Shouldn't the caliper have adjusted the slack when I bled the brakes and not back all the way down again? If that is the case, what would cause this to happen? the pedal is firm, not spongy like i would expect with air in the line

Replace or rebuild the caliper.

There might be some in and out play in the axle but I don't think it should move while on the ground. The top of the wheel should not move out when pulled on without the bottom coming too, not on a fixed rear axle.

New caliper and change the axle bearing. Not sure how a competent mechanic would miss that though. Slack between the disc and the pads should be no more than 1/16 inch.

Still not fixed! my brakes are better but the knock is still there. heres the video about 30sec in is the just of it...