Mechanical buzz is most likely caused by the power trannie or the choke, more so if the chassis is steel. Chokes, especially in a choke-input PSU can hum and buzz rather seriously.
I've run into various low priced power trannies, which hum, rattle, buzz,
and in some cases (like the infamous Hand Wound trannies), they can generate a sound like a cheap electric shaver :o Yellow_Light_Colorz_PDT_21
/ed B in NH