Take a close look at the driver and make sure there isn't any garbage behind the grill that might be making contact (eg gaffa tape)
Make sure all the external screws are present, and tight
Wrap up the handles with some cloth and see if it's them vibrating
If your cab is under warranty, stop there, and take it back where you got it
Otherwise take off the grill, remove the driver and check whether the speaker leads might be coming into contact with the cone, or any loose wadding or somesuch
As you've got two, the next step for me would be to swap over the drivers - if the distortion/noise moves to the other cab, it's the driver - otherwise something in the cab...