I realize this is an old post, but it still has relevance (for me, at least). I've mentioned this in a previous post: a buzzing vibration comes in between 2000-2100 rpm. It's pretty loud most of the time, but other times it's quieter. Turning left while it's occurring makes it more pronounced. I get it in every gear, and intermittently in neutral.
I've been dealing with this issue ever since I installed Prothane mounts, Ebay LT's and X-pipe, and high-flow cats.
I have approx. .120-.187" clearance between the steering shaft and nearest header tube on the DS, and roughly the same amount between the frame and nearest tube on the PS.
The list of things I've found and fixed that could have been causing this contains:
1. Loose tail light cover. Replaced the plastic push screws with plastic rivets.
2. Muffler hanger hanger hook on DS touching isolator bracket. Bent hook to gain clearance.
3. DS muffler (Flowmasters installed by PO at a muffler shop) rattled when I hit it with my hand. Replace with JBA axle-back system.
4. x-pipe. Fitted stock H-pipe, no change so I went back to the X-pipe.
5. Loose lower bumper cover. Replaced fasteners.
6. Spare tire cover. Removed temporarily.
7. Loose jack. installed foam under it, then secured it.
8. Vertical (shift lever shaft) spherical bearing in my Hurst shifter could float up and down approx. .010-.020" within the bearing housing. Glued it in with Loctite.
9. Aftermarket cat touching one of the heat shields. Removed factory heat shields and installed thermal barrier sheeting. I have a minimum of 3/4" clearance around the cats now.
10. Installed 4" long flex couplings between X-pipe and tailpipes.
11. Pulled the center console (it sounded like the noise was coming from under the console towards the front) and found that the connectors for the Dual head unit were sitting on the bare (un-carpeted) metal of the forward part of the trans tunnel. Insulated the connectors with foam padding.
12. A/C hard lines along PS inner fender. Temporarily isolated them with foam.
13. When I disengaged the lower shifter boot from the floor tunnel and drove it to hear what I could from under the car, the vibration was barely noticeable. When I re-installed the boot, it came back. I noticed that the rear shifter mount compressed part of the lower boot, so I shimmed the mount away from the floor by about 8mm, so that it wouldn't "capture" the boot. No joy, buzz was still there.
The next step is to get it in the air and see if I can induce the vibration/buzzing while I'm underneath. This will require a helper with a steady foot to manipulate the throttle. I've held off on this step as I suspected that with the car on jack stands the dynamics that are contributing to this buzz/vibration might be changed or removed when the car's weight is borne by the frame instead of the tires. It's worth a try I guess.
What's really odd is that everything I've done seemed to fix it temporarily, but it always comes back.
I'm just about ready to pull the headers off and put some clearance in the tubes that fit the closest, then re-install the stock motor mounts. I don't really want to do this because it shifts so much better with the stiffer mounts and the Hurst shifter.
I'm tempted to do the TSB mentioned above, but I'm not sure how well the rear part of the exhaust system would be supported by the flex couplings. I guess I could take them out, since they didn't produce the desired results anyway.
*Light Bulb!* I bet with a little effort I could pop the rubber inserts out of the mid-mounts and see if this changes anything. That way they wouldn't be permanently damaged.
As always I'm open to suggestions.
