89 bird 2.8 5 speed, engine/exhaust ping?????

About 5 months ago i had my car in the shop to have the power steering bracket replaced. when the car went in, the engine sounded fine. since i've had the car back, it's made a weird noise. i took it to a diff. garage and one of the mechanics said that it sounded like a ping. when i first start the car up, there is a very annoying pinging noise. when i rev it up, the noise gets louder and more rapid. when i'm driving on the road and i get up around 4500 rpm's, the noise stops. after about 10 minutes of driving, the noise stops. if the car runs for a while, gets shut off, and is turned back on, the noise starts again. any ideas as to what this noise is, and how i can fix it?

i looked at the bill for the time it was in the garage, and it said that they had removed the exhaust manifold. could this possibly be my problem? i am a complete newbie and i'm trying to learn as much about my car as as i can, and i figured i might as well start here. thanks

hey again yeah its the manifold I have a 89 firebird formula witch im trying to sell VERY cheap and it needs a new manifold gasket.. Its VERY loud and it smells like gas So thats probabbly your problem need manifold gasket or a cracked manifold