Rattling sound while revving: '99 Sentra

I have been noticing a rattling sound coming from my ’99 Nissan Sentra for the past few months. It has manual transmission. It seems to be coming from the middle of the car, somewhere between the engine and the front axle.

This doesn’t occur all of the time but seems to happen most frequently:
a) During low speed (20 – 30 mph), residential driving.
b) Accelerating after coming to a complete stop.
c) Slowly revving the engine at low RPM’s

I took it to a friend-of-the-family mechanic. He thought it could be a loose or faulty heat shield, and recommended a place like Aamco, since he doesn’t specialize in exhaust repair.

I took it to Aamco last week. The put it up on the rack but told me that the heat shield was intact.

Yesterday while driving, I thought that it could be the transmission until I noticed that it makes the noise while revving it slowly in neutral as well.

I don’t know what could be causing this sound or which shop that I should take it to next.

Any advice would be greatly appreciated.

Thanks.