MD80 - Wonderfully quite up towards the front of the cabin; one can hardly hear the engines starting and normal conversation is possible even during T/O. Got a mate flying the thing for SAS, or rather he was. Now he's on the 737NG and is always complaing how noisy the NG is compared to the MD. They never wore headsets in the MD except for T/O and LDG; on the NG they're always on.
However, have also had the displeasure of paxing at the second last row on a MD, right next to the engine inlet. Talk about noisy ....
As a pax, in business class mind you, I'll take a MD anytime over a NG or A320 series. Sooo quite.