Oldsmobile/Buick Repair: Engine noise, bicycle wheel, heat shields

Question
I have a 96 Buick LeSabre. A mechanic accidently dropped a bolt into the engine compartment and damaged the pistons. They replaced the engine with a rebuilt that has 62000 miles on it. The problem is that it makes noise when I accelerate. The noise is kind of a "tinny" sound and reminds me of a card being put in the spokes of a bicycle wheel. The noise is not present when coasting or when in park or neutral. The repair shop thought it was an exhaust problem but is not sure and they haven't been able to find the source. No such noise was present with the original engine. Any ideas what it could be? Thanks.

Answer
The exhaust system can be very close to the body of the car, and when the engine torques over when in gear, can make it actually touch.
If it is touching one of those light metal heat shields, it can sound like that.
With the brakes heald firmly, apply a little gas, then just a little more, to see if you can make the noise. With the hood open, you can see the engine shift in the motormounts.
Something just may be ever so slightly closer now than before.
Van