LauraLuvsHerLilTruck

Me again with my funny noises. I've got a '93 HILUX, ext. cab 2WD, 2.4-l, 22RE. Recently (just happened to be right after a tire rotation, though i don't think it's related) i started hearing a funny high metallic whirring/cycling sound, very much like an old-time hamster wheel going around real fast, from underneath the truck... at puzzling but regular times: ONLY when decelerating or coasting, and only with clutch engaged. That is, i don't hear it when i'm applying gas. And as soon as i push the clutch in, it stops. I got the mechanic at my last oil change to listen to it (wasn't at the time ready to spend time and money on a fix, when the noise is so subtle it doesn't seem dangerous)-- he listened underneath and said it was coming from the drive train somewhere where it engages with the clutch, and didn't know why it was only happening when decelerating. (Exactly!) He said not to worry 'til it got louder, nothing was going to pop apart, it all looked fine.
It seems illogical that it only happens when NOT applying gas pressure (so-- when things are loose and not pushed), yet doesn't happen when i disengage (push in) the clutch (again, when things are not "under load").
So for two reasons, i wonder if anyone knows what this whirring sound could be: 1. I am going on a 1000-mile or so road trip out of state next week, and would like to know what might happen; and 2. It's such a mystery, i'd like to make sense of it.
Thanks for any help or similar stories you can share.