Engine Knocking

I own a 2004 Nissan Maxima. I am noticing that after the engine is warmed up, and I depress the gas pedal, I hear a tapping sound from the engine. When the car is rolling at about 35 mph, and I slowly depress the pedal, the tapping is constant until I depress the pedal harder. Can anyone help me out with this? Has anyone experienced this at all? That sound is very irritating. It actually sounds like valve tap. I took it to the shop, and (as usual), the service manager after stating that he hooked the car up to a computer, said it they could,'t find anything worng, although the technician that also rode with us said it sounded like valve tap. Any suggestions, or other info would be highly appreciated.

Bstevens
VA

I had a knocking noise that lasted for a few months... I have a 5 spd, it sounded embarasing when I let the clutch out. A few weeks ago it turns out that it was a faulty bearing in the transmission: it exploded while driving one day. The car was still under waranty and the claims inspector said that it wasnt my fault so they took a 3,500 bill for a fully rebuilt tranny. =o) Its good now.