If this is happening while you're changing the volume from your remote, the noise be caused by the little motor that is moving it, if it just happens with the remote.
I've seen some new upper-end units do this, while some cheap ones don't. It depends on the design. You could ask the company if this is normal while using the remote. If it did this since new, it just may be its design. The motor doesn't do anything (add noise), when all is well, while playing music at a fixed level.
If it does it while changing it manually by the knob, then the volume potentiometer most likely is the problem. It could be adding some noise that may not be really noticeable now. But, it can worsen, if this is the case.