I've spent up to four hours flying in MD-80s, and for sure, the closer you are to the rear of the cabin, the louder it is. Of course, with tail-mounted engines, the noise level in the last few rows is much higher than at the front. Also, MD-80s and DC-9s tend to have alot of resonance in the airframe due to the engines, which are not of the latest design.
If speaking about Stage III compliance, I discovered in a recent post that the MD-80 is indeed compliant. However, I live near MKE, and when MD-80s take off, they ARE rather loud. It is definitely possible to tell the difference between an MD-80 and a 757 or A320 in terms of noise - the latter two are much quieter.