Banging pipes.

My townhouse suffers from banging pipes. It seems to occur randomly and it doesnt matter whether water has been running or not. It can happen in the middle of the night. I added a water hammer unit to the cold side of the main water supply, but the noise persists. I did replace the pressure reduction valve several years ago and that solved the problem–for awhile. I about ready to replace the PRV again but would appreciate any suggestions at to alternative solutions to this noise problem. Thank you very much.