screeching/grinding sound when coasting at speed 2003ish Cannondale six13, all Ultegra.

Frequently, but not always, when I'm coasting at a fast pace, usually down a hill, the bike will start making this hideous screeching/grinding noise.  It is silenced if I rotate the pedals a bit, but might start up again immediately.  I'm reasonably certain it's coming from the bottom bracket.

My questions are:

If it is the bottom bracket, why would it be making this sound?   Can the existing one be serviced to fix this?

If it can't be serviced, should I just get a new bottom bracket and install it, or could it be something else?

Since I cannot reproduce this off the bike, what should I look for if I take things apart?

Thanks!
I think it's impossible for it to be coming from the bottom bracket while the pedals are not turning.  More likely it's coming from the rear hub.

I'd first check whether the spoke guard (if you have one) has come loose and is rubbing against the cluster.  And inspect the area behind the cluster for any piece of trash that has gotten in there.  Failing that, you probably have a problem with your freehub.