JonGS3

This problem started towards the end of summer but I kind of put it in the back of my mind since fall was approaching. I figure I should start diagnosing now so it's hopefully resolved by the time we get through winter.

Here are the symptoms (all with the A/C turned):

-When stopped in park or in drive, I can hear a low rumbling sound in the cabin along with a mild vibration through the brake pedal. If I'm in drive, it goes away once I start accelerating and reach around 1700 RPM. If in park, it goes away if I rev the engine up to about the same RPM. It seems to be worse when the car is stopped in drive vs. when it's in park.

-The rumbling is louder when there's a greater difference in ambient vs. A/C temps ie. It's more pronounced when it's really hot outside (like mid upper 80s and above). It's not as bad if it's around 80 or below but is still there. (I haven't tried running the A/C since it's been cold out but I'll do that the next time I drive to see if that makes a difference)

-It's intermittent. It will do it more often than not but sometimes when I come to a stop it doesn't happen or it won't happen at first but then will start to do it if I've been sitting at a light for awhile.

-I can't hear any vibrations or noise from the engine bay. I was in Florida over Thanksgiving and looked at it with my dad (he's an aircraft mechanic). Neither of us could pinpoint anything in the engine bay despite being able to hear it in the cabin.

-It doesn't seem to do it when I'm driving at speed or if it does I can't hear or feel it.

My A/C works and is properly charged. It only does it when the A/C is turned on. The compressor clutch seems to be functioning fine. I can see it spinning on the compressor when the A/C is on and it stops when the A/C is off.

My first thought was the blower motor but isn't that still running the fans even if the A/C is off? Is there another mechanical part in the dash that only runs with the A/C that could be causing this?

The problem first started around the time I replaced a bunch of missing hardware that connects between the lower bumper, bottom engine cover, and wheel well liners. I had to force some of them to line up. I thought maybe them all being properly connected was causing one or more panels to resonate at lower RPMs. It still did it even after removing some of the hardware to allow more flex between them so I don't think that's the issue and even then it wouldn't cause a vibration in the brake pedal.

Any ideas?

DMPesso

Have you checked your motor and trans mounts? I know if they are bad anytime you come to a stop with the car in drive the RPM will drop a little and it’ll start shaking.

JonGS3

I did the trans mount about 50k miles ago. The trans pan is starting to weep a little so I figured I'd change the mount out again when I do the pan gasket.

I had a mechanic look at the motor mounts the last time I had an alignment. He said one was fine and the other was a little worn but not an immediate concern. I'm going to have to take it in to do the oil pan gasket sometime soon and figured I'd have the mounts changed then.

Generally the car is pretty smooth when stopped in drive especially considering it's at 188k miles. I noticed it's most smooth when it's in the 70s out. When it's colder or hotter with the A/C on then it can vibrate a little as it idles at different RPMs. The rumbling is weird though...I've tried to replicate it with the A/C off by holding the throttle at different RPMS in park but it's always smooth.

sburs4

At 180k your at that time again for a major service (timing belt/water pump/seals/etc.)
Also the 2jzge (and 2jzgte + 1j) is known to throw a crankshaft pulley at higher miles. I hope this isn't the case but Id check for a minor wobble in that pulley (which could be the nvh on pedal) especially when the ac switch is thrown and your high speed idle kicks on

JonGS3

The person I bought it from had the timing belt done by Lexus at about 132k miles. Good point about the crank pulley though. I'll check it out when I can.