Towing troubles

We have had our current tow car since 2004 (Mazda 6 Estate 54 plate) and used it to tow two caravans. Our current van bought new in 2006 (Bailey Ranger Series 5 470/4) with no real problems. Until this year..

Returning home from Moreton in Marsh this summer we noticed an unnerving noise from the rear. I could not figure out what it was but decided that the weight distribution must be wrong or the nose weight was wrong. I decided to replace my ten year old nose weight gage. I bought the Melenco one.  We have just returned from our last trip of the year and had the same noise from the rear going out and returning very unnerving and we were travelling very light for us. I thought the suspension had gone (dampers and or springs) in the car and had this checked over by a mechanic who said it was fine, no problems.

I know there is something not right. I have read the club’s advice re suspension but not really clear what to do.   Would appreciate any other advice please.