If it has always done it, no problem.
It's because of the high speeds used nowadays.
The speed varies as data is transfered, or proccessed.
It sometimes occurs on systems that not got a solidly built mount for the drive. Therefore, any imbalance come out as vibration. As a car wheel does when out of balance.
I had the trouble with one drive, and took it back, but was OK on their machine, when tested.
Hope it helps.