Teething troubles part two

You know the scenario - everyone with a motorhome has been there.
There is a constant knocking, tapping noise coming from somewhere in the back of the van as you are driving down the road.
Sometimes you are unable to stop and investigate and just turn up the volume on the radio for the next 100kms.
Sometimes you can stop, and then proceed to empty and re-pack every cupboard, drawer, wardrobe and locker in the vain hope that the banging will stop.
You then sit in the drivers seat, start the engine and drive off, praying that the banging noise will have stopped - but no - there it is as loud as ever. Nerves frayed and now on non-speaking terms with (the best) navigator (in the world) after all the arguments about who wedged the cups in and "did you know the salt pot was rattling against the olive oil?", you think - WHAT IS THAT NOISE??
This was us for the last four months.
Sometimes the banging was there, sometimes not. There did not seem to be a pattern to when it happened - we did nothing different for a month and still it came and went.
The noise was as if a sheet of plywood had been put on top of another sheet and were vibrating against one another.
We put felt pads on every wood/wood interface we could find - seat lockers, cupboards, doors, wardrobes, drawers - all to no effect. The noise still kept coming and going.
Things came to a head in Humilladero in November on our Andalusian Cities and National Parks tour.
We were leaving the site in Humilladero using the very quiet, but very uneven access road. The uneven road surface made the banging start, so we decided to drive up and down this road, however long it took, until we had definitely identified where this noise was coming from.
I drove up and down the road for a good 15 mins whilst Glyn was on her hands and knees in the dinette behind me trying to identify exactly where the banging noise was coming from. She finally said "it's coming from the water tank" as she could feel the vibration on the panel that enclosed the water tank under the bench seat.
We had a new water pump fitted in France in July 2013, so I immediately thought that this was the cause, and that it was somehow vibrating against the side of the water tank. Having got this far we decided to take off all the seating cushions and remove the panel that covers the top of the water tank (that acts as the seat base) and see if we could replicate the noise.

View of the tank and the panels that surrounds it

As soon as the panel was removed the cause of the problem was apparent. The water tank was full of 120 litres of water (just filled at Camping Sierracilla in Humilladero), and the pressure of the water was bowing out the side of the plastic water tank so it was touching the panel that surrounded it.

View from above showing the bowed-out tank touching the panel

View from the side showing the bowed-out tank touching the panel

I was not convinced that this would make such a loud banging noise that we had been hearing, but a drive up and down the road again soon proved me wrong.
We emptied out half of the water (60 litres) and again drove up and down the road and...... no noise!! The tank  was now less deformed due to the reduced volume of water and was not now touching the panel.
This explained why the noise was so intermittent - it depended on how full the water tank was.
After we got home a quick visit to our Burstner dealer in Murcia where we bought Cervantes saw them agreeing with us and ordering a warranty replacement from Burstner in Germany. This was a great relief to me, because I could probably stop the noise myself, but the main fear was the water tank bursting and 120 litres of water flooding the floor of Cervantes - that would take some sorting!
The tank was duly fitted in February 2014, and at the same time the first annual habitation check was done, which involved testing all the services to the living part of the motorhome such as water supply, heating, gas supply, electric supply and drainage. Also completed was the guarantee check for Burstner which consisted of checking all the seams of the bodywork for soundness and for signs of water ingress.
Happy to report that Cervantes is now fully functional and hopefully all the teething troubles have been found!

The new tank on the workshop floor - just out of its box from Germany

New tank installed with no sign of bowing

As good as new - long may it stay that way!