Thought I'd update - as part of the water change today I moved the filter outlet so that it is part in the water and part out - and guess what - it's now near silent!
Yeah, it's Errol. I did say to OH that if it was annoying me what on earth did Errol think of it??!
Although, I do think some of my annoyance about it came from the fact that I was having to sit in my office to do a particularly pointless and mind numbing assignment for Uni!!