Background rumble on ....'91 250

[Jules]

I have a rumble on my '91 250.......

At rest, engine idling, engine revving, clutch in or out: no noise, everything shipshape.

Pulling away from rest, there's a background rumble (not a rattle) which doesn't change note, it's not speed or revs dependent. It's there more or less all the time, sometimes a little louder, sometimes a little quieter. Seems to be worst when slow filtering through stopped traffic. Pull the clutch in at low speed and it's a little noisier. Of course, after 20mph or so, I can't hear if it's getting worse or if it's speed/revs dependent because of windnoise.

It seems to be coming from clutch area, so I assume it's clutch housing, bearing. FYI, I believe the bike was left untouched for a long time. 

Any ideas? Is there a MAGIC oil additive to quieten this (the oil/filter is new)?

Cheers

CORRECTION: When it's hot and idling and the clutch is pulled in, there's a slight rumble.

[andrewsw]

I have a rumble on my '91 250.......

 Seems to be worst when slow filtering through stopped traffic.

sound reflecting off the adjacent cars makes it seem louder. other than that, no help.