DIYbanter - Noise in the pipework

Okay, Friday's here, so time to start thinking about what bloody jobs to do
over the weekend. One thing that has been annoying me for a while (okay, it
anoys her, then she annoys me) is the following bit of noisy pipework...

When running a hot-tap to fill the bath or basin in the (upstairs, single)
bathroom, after a few seconds of the water running we hear a low-pitched
"drone" coming from the pipe-work. Now, this only seems bound to the
hot-water, for reasons that quite baffle me (I was thinking along the lines
of the valve leading into the header-tank being worn - but surely that'd
have the same effect for hot and cold water?)

The "drone" continues for maybe 10-15 seconds, when it gentley subsides over
a period of 2-3 seconds, never to return until you re-run the tap. The
"drone" frequency appears constant and isn't a million miles away from the
50Hz hum / alarm of an old clock-radio. The "drone" appears to be coming
from the attic space where (from memory) there's a bunch of piping heading
to and from what I *think* is a single header tank.

Indirectly heated hot-water/central heating system, BTW.

Doesn't seem to be affected by the kitchen taps being run, or the cold taps
from the kitchen.

Any suggestions as to where to start looking?, being the weekend I'll
probably be trying to fix this myself, and the opportunity to run-tap,
run-up-ladder, jump-around-loft in the 15 seconds that I have to determine
which pipe is vibrating gives me a little concern.

All suggestions gratefully received.

Regards

Mike Dodd