slight knocking in h23

I bought my prelude a few months back and ever about a month after I got it I was replacing the stereo and had to drive around for a few days without it. I noticed a slight knocking in from like 2300 rpms to 3000 or so, but I didn't think much of it. It is still there now and I'm begining to worry about it. It only happens in the first few gears. 1, 2 and 3. 4th and 5th I don't really hear it. It's very faint and can't hear it unless I turn the stereo all the way down. Also it only happens under load so I can't really check to see where the noise is coming from. Does this sound like a really bad problem, if so how would I go about fixing it. It's a 92 SI with 101k on it.

Thanks for any help,
Nick