Strange Noise From Transmission Or Exhaust Pipes

[Chipofftheoldblock]

Has anyone experienced a noise (typical?) when letting off the accelerator at freeway speeds?

I have a 97 LX450 with 68k miles that I bought in June. I have been becoming more aware (not that it has gotten worse) of a noise either coming from the transmission (4th gear of the automatic trans), differential, or exhaust,,, only when decelerating from 70-70+mph. The best way to describe it is the sound that is made when your vehicle goes out of the lane onto one of those grooved pavement warning spots on the edge of the pavement on interstate highways....but not nearly as loud. While cruising steadily, accelerating. and speeds under 70mph, it does not occur.

Could it be just part of the nature of the full time 4wd?

Or...fluid related (low or old)...Has anyone had their transmission drained and flushed as part of preventive maintenance?

Or possibly the timing chain/gear?

Throw me some feedback if you have any ideas.

Chip