I have a 1994 honda prelude type S manual.?

I'm hearing this weird noise as if you were to take your fingernail and tap it real fast against wood. It happens when i accelerate, but when i'm at a red light you dont hear it. It sounds like it's coming from the engine, but I dont know much about cars. My mechanic use a stethoscope and listen for the sound and he told me it was my alternator, but I just bought it. The noise sounds like it's coming from the right side of the engine where the oil cap is on my prelude. The weird thing is that the car still drive normal. Any body know what the problem is. I'll really appreciate it. ;)