Why is there a whining noise when you take your foot off the gas pedal?

On my 99 Montana it turned out to be an exhaust leak in the catalytic converter. This is additional comment to above question about a whine noise from another person.

I have no solution yet, but overdrive lock up stopped functioning in "D" and it now "whines" with change to engine speed, especially noticable going up and down hills as load changes. This noise starts after about 20 minutes of driving when everything gets up to temperature. Putting the transmission into "3" out of "D" and it will quieten down a lot, which takes the lock up overdrive out of action. Yes, transmission still runs ok and has for 8,000 plus miles. Seems ok but I suspect it may cause problems or transmission failure eventually.

PS: Transmission has 177,000 miles at this time.