I feel like I am chasing my tail !

I have been chasing a vibration for a long while and replaced all my U-joints, bushings, motor mounts, transmission bushings, driveline geometry... each time that has fixed some issues and led to me discovering others.

The protagonist is a 1969 with a 400 SBC and a T5 transmission.

I am experiencing a rumbling vibration that seems to be linked to engine RPM and not speed.

The 400 SBC is nearly new (less than 1000 miles on it). There was no noticeable vibration when I went for a test drive when it was in the lux-o-barge with the TH350 I pulled it from.

I am quite sure it is not related to the new flywheel I put on there. I once had a 400 with a 350 flexplate and the vibration I was getting was very different from what I am experiencing now.

So, I am ruling out the rotating assembly, U-joints, drive shaft balance, transmission bushing,...

The vibration seems more like a rumble that echoes the exhaust note. It seems more pronounced on acceleration and less intrusive at high RPM cruise.

Here are my thoughts.

I have noticed that the base of the shifter is touching the shifter console. I think normal engine vibration is being transmitted from there throughout the cabin.

I also noticed that on harder acceleration, the shifter yanks over significantly toward the driver. (This is a new rubber trans mount.) I am speculating that this twist is aggravating the vibration transmission. I am betting that, as there is already very little clearance between the T5 and the trans tunnel (so far as I recall), on acceleration firm contact is made transmitting the rumbling vibration.

The rumbling vibration as I said echoes the exhaust not - this might also be enhancing the feel.

My solution is to try a low profile (1 inch instead of 1 3/4) polyurethane trans mount. The way that shifter was coming over, it could well be that my new rubber trans mount is torn. (My motor mounts are new.) In addition to preventing somewhat that banking, it should lower my trans and shifter away from the body.

In any case, I ordered the low profile trans mount. We shall see if that makes a difference.

I welcome any thoughts in the meantime - stay tuned

 

Not actually experiencing this vibration. almost impossible to give any advice. other than, do you really expect your 50 year old Vette to be as smooth as your wife's new mini-van???

 

have you tried moving the shift handle to the right with a small spacer in the shifter mount

 

Ummm, yes ??? Point taken. Hm, I guess I would expect it to be as smooth as my 1973 Chevelle.

To be fair, a buddy of mine in the business says, it's not that bad. I guess I am a stickler.

 

Is the sbc 400 and the T5 the protagonist or the antagonist?

 

Your shifter shouldnt be moving much if at all. Check your motor mounts and transmission mounts as well as your mounts themselves. Rumbling vibrations are usually associated with a ball bearing as well. YOu may want to look in thatt dirrection.

Does the rumble happen while in nuetral? When you feel the vibration and you step on the clutch does it go away?

 

You mentioned the flexplate, but not the Harmonic balancer. Both need to be correct for the external balance 400. Also confirm the pinion angles are correct.

 

Well, after you change the tranny mount let us know what happened. I really do suspect that it's the flexplate/harmonic balancer combo. Do you have the old tranny? If the rumble doesn't go away then I'd put it back in to check.
AND -- I would expect it to be smooth so don't accept that it won't.
My richmond 6-speed is smooth as glass until warms up and starts rumbling....and even then it only make the rumble racket at idle....it's smooth at speed and while driving around.

 

check motor mounts..they will look ok until you jack up one side and other rubber seperates

 

My rumble/rattle comes from the back only in a narrow RPM range usually when coasting down in gear. Sounds like a part of the body making the noise. What it is not is: gas tank, exhaust system, motor mounts, shocks, or front or rear suspension (inc. springs) as these have all been rebuilt or replaced. Suspect body mounts but I am going to live with it now as there are a few other things to work on first.

 

Forgot about this: We had a 2003 Corvette and I had won a Billy Boat exhaust system at a rally. I got the bright idea to install it and hated it from day one. Too loud with a terrible rumble/drone between 1700-1800 RPM's which made cruising miserable.

Could you have a resonance in your mufflers?

 

My original Muncie would eat up input shaft bearings. It was a cyclic rumble and varied with engine rpm.
Rear spindle bearings also rumble/howl when they go bad, varied with wheel speed.
TT

 

I think we might be on to something.

I don’t think it answers everything but presumably the transmission mount can’t do its job properly

 

in the third video you can see a bolt with a bit of orange on the tip. Those need to be tight as well, you should checlk all of them obviously

 

It looks like the 2 bolts that come down vertically through the crossmember are loose. It wouldn't wobble like that otherwise.... unless it's cracked? Below is an image for a 68 but should be about the same:

 

The side bolts aren't even finger tight. I think it is unlikely that vibration shook them loose. My guess is that I simply forgot to tighten them.

I suspect that tightening them will improve things, but not solve my problem

I ordered a low profile, polyurethane trans mount. The idea is to move the T5 away from the trans tunnel and shifter console to avoid conducting normal engine vibration throughout the cabin. Presumably the polyurethane will more positively locate the T5.

So stay tuned...

 

U mentioned that the shifter is hitting the front of your console......I'm replacing my shifter NOW, and it expressly states to "Shim as necessary to maintain a minimum of 3/8" clearance between the shifting handle and the console!!

 

Yeah - I don't have that . I mean it is actually preventing the console plate from being screwed down flush...

The new trans mount and tightened bolts should make some kind of a difference - hopefully a sufficient one.

 

The way that it is moving in your videos - you'd better tighten the vertical bolts also. Those the ones that would prevent the "tilting".

 

Agreed. This really needs a good going through