to check if its an engine mount, start your car, pop the hood. push the engine to the left ( not to hard out) if it still rattles then push it to the right. if the rattling stops then its an engine mount. if it continues to rattle then look at your balancer. and to check if its the balancer take your drive belt off, then shake the balancer and if its loose and makes the rattling noise then thats it.
ive had both problems with my vt had to change balancer and engine mounts, and rattling sound was fixed.
and for a mechanic to change your engine mounts, at work the last one i changed cost 100$.
but mate i may be wrong with any advise here, and hope my tips can help you out.