Following photo shows a residential air conditioner compressor mounted on a flat roof. It sits on two wooden planks, and each plank is supported at two points by a stack of three pieces of what appear to be rubber pads. Can we do anything here to improve the noise and vibration isolation of the compressor?
Perhaps. Depends what you have now, which is not terribly clear from the picture.
Also depends if those stacks of pads are carefully located on framing already or should be moved so they are supported on framing rather than using the roof decking for a speaker. That, alone, could make a big difference.
Determine exactly what sort of "rubber pad" you have now, and you can find out if pads (or other vibration mounts) with better vibration absorption are available (likely at more cost) and swap them out. You'll need to make sure that the area of the new pads (or weight rating of the other mounts) is sufficient to safely support the weight of the compressor/condenser unit. Pads normally have a "weight-per-square inch" rating so by making the contact area greater they can support more weight.
There could be some benefit to moving some (not all) pads to go between the compressor "feet" and the wood, rather than having the wood in hard contact with the compressor base, but if they are bolted to it that might be tricky to manage mechanically while still letting the pads work. There are bolt-attached vibration isolators of various types that might address that if needed.
Probably would be wise to involve a refrigeration professional in the process so you don't do something inadvertent that breaks a line, or drop the unit. At least think the whole process through along with every possible thing that can go wrong a few times before actually attempting it. But experience can be worth paying for to reduce the costs you might pay while gaining experience yourself.
