Were going to need more information than that. Maybe post a video.

Any problem becomes expensive if not addressed or improperly diagnosed, and I speak from my own experience on this. Occasionally you catch a break and find a bolt just needed tightened.
Is it worse under load, winding up, coasting down, any time? When did you first notice, and was anything done to the car just prior? (Oil change, visit the drag strip, jump a curb?). I’m also a stock 90 gsx owner. You’ve come to the right place for help; this forum rules!

How many miles on the car, and how long have you owned it? When was the last major service, & what was done?

Jump to the 16:50 mark to see what I imagine may be going on with your engine also.........But I hope not.
I am a stock 1990 GSX owner also!

I just turn the radio up when this happens to me ! As others mentioned, can you get video/audio of the sound?

That radio fix works for so many problems (except a broken radio).

Loved watching that video, btw. I'll send it to my muscle car friend who's all into big block V-8 (well, yeah, their cool too). I think the valve train in the 4G is just ingenious. Such low moving mass compared to a cam in block V-8. I hope KeyFlo_dsm isn't having that bearing failure in his car. What typically causes that, I wonder. The fact that it's only noticed in a certain rpm range isn't what I'd expect from a bad rod bearing, just based on the mechanics of it. If it happens even when the car is not moving, rev it with the hood up and listen to where the sound is. That should give a clue.