I have a 90 GTR has about 140,xxx Km on it. Was leaving work and I heard a tapping noise. It rises with my rpm and is noticeable more when windows are down. It only occurs when I'm at light throttle and for 2k-4k range. I used the stethoscope and doesn't make the noise when not driving. I thought it was maybe bad gas but I ran premium through it and still the noise is there.I'm confused at how to trouble shoot this further. Oil pressure is good and Oil temp stays low. Possible causes?

You may also want to post this in the RB forum as you'll probably have better luck with a quicker response.

Unfortunately, I can't help you here, but there are a couple of people that may be able to should they pop in.

Hope you find out what's wrong!

Could be knock. Put a scan tool on it, or hook up the stethoscope when you are driving to listen to the engine.

Detonation sounds a little like marbles in a can.

Turned out to be a rod bearing. Rebuilt it since it backed out the oil pump screws and pretty much killed it. Runs awesome now and dont have to worry about the same issue since I used R33 crank and I was able to score some new tomei oil parts(pump,baffle,pick-up).