'03 M45 with "Thud" after heavy acceleration... [SOLVED - motor mounts]

[Tim/M45]

Hi all,
After WOT start from a stop there sometimes is a thud from under the car when I let off the gas. At first I thought I rolled over a pot hole. After a few times I started looking in my rear view to see what I hit, but only see smooth road. Sort of feels like a bad engine mount but I'm not sure. I cannot see any indications of this on the mount. Somewhere I read about an issue with the differential potentially doing something like this but I don't recall the details. Any suggestions are appreciated. Thanks.

[Ilya]

Do you let off the gas quickly or gradually? I could see the mount possibly being the issue if it was quickly, but if gradually it's probably something else.

[EdBwoy]

I am betting on the motor mount as well. And at this age, rubber has things happening to it.

Since we have the luxury to have a cable operated throttle, I'd try to check this with the car parked but running. Give the engine a few snappy throttle blips and see if the passenger side of the engine rises higher. It might be difficult to hear any clunk with the engine screaming at you, so you could place your hand or other sthethoscope/alternative to hear any heavy noises as you let go of the throttle.

Another more certain way is a little more time consuming. Go under the car, undo the 2 bottom nuts for the motor mounts and Jack up the engine. Within an inch or 2 you should be able to see if there is a loose one.
*remember to remove nuts, THEN jack the engine otherwise it's a destructive test.

[Tim/M45]

Yes, it only happens when I let off the gas suddenly after WOT start. Doesn't always happen, which is why I was not convinced it is a mount. I will try jacking up the engine the next time I do an oil change.
Mounts look to be almost $400. Well, it's been a while since I've dropped any $ on this car.
Thanks for the input.

[EdBwoy]

I think you could do the manual throttle blip under the hood quickly and simply... or just have someone in the driver seat while you watch.
Or just have a camera recording while you do the throttle yourself from the driver seat.

Either way you go, please let us know what you find.

[Gene Ln]

Other alternatives could be something drive shaft related, or even axle related, when this happens can you get a "feel" on where this is happening? ie under the front of the car, the middle, or the rear?

[Tim/M45]

It's hard to determine where the Thud is coming from. Sort of feels like it's under my seat, but maybe from front end.

[Tim/M45]

Update: I did the tests EdBwoy suggested and sure enough, the driver side mount is shot. Found an OEM set on Amazon for $270 so placed the order. Will get them installed when they come in. Thanks for the input.