pulsating PSU not the fan?

[asiandunn30]

thermaltake toughpower 750w modular....does psu make a pulsating vibration? when i turn on the PC u can feel the pulsing vibration coming from the psu on top of the case. is this normal?..its not the fan on the psu or any other part of my pc..when only the fan is on in the PSU it doesnt make the vibration but when i turn on the pc it seems to make this pulsating vibration which vibrates the top of the case..