Same problem, used stainless "P" clips fitted with rubberised inserts as available on eBay to secure the exhaust hose, and like Lyra also had to replace one of the plywood mounts. It's worth taking a look at your heater exhaust while in the lazarette, as on mine the outer end of the flexible stainless exhaust hose where it connects to the hull skin fitting was corroding. Eberspacher agents offered to replace the stainless hose, and were probably having a laugh as they quoted about £200+, and VAT etc., but the hose is available on eBay at around £15 per metre.
First encountered the thumping noise under foot on my friends B37, on our way back from the Whitehaven Carnival a few years ago, and wrote about within this web site, but can't remember what it went under.