I've got a weird vibration going on. It only happens around 55 mph and it isn't constant. It happens about once or twice a trip. It's a faint, high frequency vibration - kinda like a low buzzing feeling through the floor, seat, and steering wheel. It only lasts a few seconds then goes away, then sometimes comes back after a few more seconds, then goes away and doesn't happen again. I'm really hoping it's not TC shudder. I'm going to be replacing the struts this week because they're pretty worn out. Could that be a cause? Any input is helpful!

 

Exhaust/catalytic converter shields

 

Wouldn't that be more of an audible rattle? It's more feel than hear.

 

True it would be more of a rattle, but also very easy to check & rule out.

Might not be a loose shield really, but more of something bent or mis-aligned so that it occasionally touches the car's floorpan.

 

Checked the shields and both are tight. What should I look for? I feel it through the seat, floor, and the steering wheel. It comes on suddenly, and goes away just as abruptly, all at 55 mph.

 

Look for sufficient clearance between the exhaust pipe, heat shields, & floor pan. But it's hard to define what's "sufficient".

First, think about any conditions that make it happen. You think it's random, but things like being on the gas vs. coasting or engine-braking. Anything related to steering right or left? Even road noise (surface texture of the road)?

Can you make it happen or make it stop by shifting to neutral & coasting?

Look for any heat shields that look like they're bent - for example being hit by debris from the road. That might allow something to touch while driving. And there's more heat shields screwed to the underside of the floorpan along the whole length of the exhaust - not just the 2 around the cat.

Look for things that might come closer together when the exhaust pipe grows longer as it heats up to operating temperature.

I had something like this and it drove me crazy for a few weeks. As it gradually got worse, I figured out it was a cracked exhaust pipe that was opening & closing with the throttle. Crack was nearly below the shift lever & it was very tiny. It had to grow bigger before I recognized it as an exhaust leak - it sounded more like a buzzing at first.

And still... Sounds and vibrations are real difficult to figure out over a forum. An experienced mechanic riding along might recognize the sound as something completely unrelated to exhaust.

PS: I just noticed your other post about the intake manifold. Examine all your work around that area.

 

It seems to happen when it's a nice, easy throttle. Like cruising. It's not the road surface. I went over the same spot twice and it didn't happen the second time.

I never thought to try throwing it in neutral. It hasn't done it for a few days so I haven't had the chance to try it. I guess it's a good thing it hasn't resurfaced. YET.

I'll look all along the exhaust and see if I notice anything.

Concerning your P.S., I've compiled a whole list of things to test/check. Hopefully I can go over them this evening. I'll let you know what I come up with.

 

Update: caught it doing it again yesterday after not doing it for nearly a week. I remembered to throw it in neutral while it was happening, but it still vibrated when in neutral.

 

Then you can rule out things related to the engine, like alternator, AC, or PS belts & brackets.

I think you have also ruled out exhaust pipe vibrations, because those would go with engine RPM.

Transmission is trickier, because big parts of that are still spinning at road-speed.

 

I wouldn't think suspension would be a culprit because the tires, ball joints, and struts are all brand new. I don't think it couldn be the TCC because it would stop once I shifted into neutral. Gah, I pray it's not the tranny, but I don't know what else it could be at this point. What gets me is how random it is; it only happens for about 5-10 seconds and it stops and doesn't come back for a while, whether it's a couple of days or a week.