you have a ps2 with 8.4 volt battery in the tender,if your bcr doesn't hold a charge could effect your sound!
http://mthtrains.com/sites/def...loded/20st11553e.pdf
check out the PDF file, shows all your engine parts and battery,i would try a 9 volt alkaline battery first for a quick check ,if sounds return to normal, bcr or charging circuit is your problem!
good luck
Alan