I also get this too in my my01 rx2.5, only happens in fifth gear if I just tap the accelerator now... my mechanic can't really figure it out either... he said he's seen in a few liberties and said it was something to do with the hillstart brake hold getting a little stuck?
When I took my wheels to get realigned the tyre guy also picked it up and said it might be something to do with the brake sticking also...
I've just say what the other gents have said...work on your transition between the gears (possibly let the clutch slip a little?
)
I've managed for a few months now without hearing it too often, but will have the mechanic look at it again next service.