its turned out to be the vacuum intake ( i think) on the vacuum/waste tank,its like a sink plug hole!!
anyway through my vaste experience of porties

i eventually after 3 hours of panic, figured out that any air getting into the waste tank would be bad.I then got the wife to put her hand over the holes (sink plug) and by je*s*s it pulled

I now have to figure out how to fix the water temp

any ideas

Ryan