Myryad MA240 power amp pops when shutting down.

hi, I have 4x myryad MA240 Amplifiers (running bridged as monoblocs) and one has started to pop when the relay shuts off as its powered down. the other 3 amps don't and this is a new issue for the one that does. Any ideas what is causing it? could it be the relay is worn and needs replacing? or could it be something else? otherwise it works fine.