I guess I could leave things run but I'll go along with the plumber on this.

He spotted a pipe was out of line with its clips (paint marks) and the theory is that it may have been dislodged by someone who was in the roof recently. The idea is that the "compressor" was amplified sound as the pipe was hitting a skirting board.

Time will tell but maybe a quick cheap fix on this one with him just pushing the pipe back up.