.....tap, knock, rumble, hiss, pip or tick.....?
If it has a fan (some do) do the blades rotate without hitting anything? Is the fan working ok (i.e. bearings)?
The compressor is physically isolated from the metalwork by rubber bushings. One may have failed. Similarly, a lot of the freezer pipework is left 'floating' in the air - is any part of it touching the chassis/frame? Simple massaging (slight bending) of the pipe work will shift it out of the way.
If the noise is from the compressor bearings then you need a new compressor.