I think the APU is what renault refer to their fancypants air drier that operates on quite sophisticated electronics. It may be that some sensor info from somewhere else is causing it to open the exhaust if you’ve had a new one fitted and the same issue.
If only with trailer I would be looking at the trailer control valve also. It doesn’t sound like a compressor fault unless a compressor fault stuffed the last drier and has now stuffed the new one, carbon build up is sometimes a problem.
Can you hear it ■■■■■■■ any air from the drier exhaust when uncoupled?
Hopefully Neversweat will be along, who is a Renault tech, and the issue might be familiar to him.