RPM appears to be the factor here, as well as engine load; accelerating in any gear from 2500rpm on up causes the noise, but cruising at a steady 3000 in 2nd or 3rd, for instance, usually doesn't cause the noise. Location is devilishly hard to pinpoint, because I can't seem to reproduce the noise while the truck is at rest. I'm just trusting my ears while moving down the road to say it's from the front end, but could be anywhere from the engine to the gearbox; at this point I can't be more precise, but I'll try to figure it out with someone else driving and see if that helps. Thanks.