Weird ghost sound coming from under car???

My 420 SEL is making a weird noise that sounds like a ghost coming from possibly the bottom and rear of the car only when you are driving, hit the brakes to a stop, and then hit the accelerator. The kicker is that it does not happen all the time, so my tech could not hear it, but it is happening a little more frequently now.

The only thing that I had done a week ago is I had my tires rotated and the guy said that since the last time your tires were rotated, someone did not tighten the bolts all the way on one wheel, your bolts became bent, so we will have to replace the 5 or 6 bolts. They did it for free so I said no problem.

4 Days later I am getting this weird noise sporadically now and then after I let go of the brake(not exactly sure where it is coming from). Is there something that could have happened the to the brakes, wheel, rotor, disks, etc., from replacing warped bolts (causing from being too loose).

My Tech checked the brake fluid and the tranny fluid is fine. Alhough it is not applicable, I got new plugs and "suppressors".

It is possible that it has nothing to do with the wheels. Maybe someone had heard a noise like that and had a different problem.

Any feedback would be greatly appreciated.

How about the rear? Differential? Tranny ?