99 cougar idle problem

Friend of mine has a 99 Cougar automatic tranny. apparently things are fine when she is accelerating but then when she lets off the gas to slow down, she hears a rattling noise, her rpm's won't drop and she has a burning rubber type smell. eventually the rpm's drop after she stops but the rattling will sometimes persist. she said that when she shifts the car into neutral, the rattling will be heard for a second then stops completely and the car is fine. any ideas as to what could be causing this?

the idle air control valve (IACV), costed me $85 for the part, and about 15 minutes to replace (very easy repair)

can i assume the problem started when it started getting chilly outside?