not a plumber by any means but I had a similar problem for about 6 weeks, done loads of things suggested to me but to no avail, I just happened to mention it to an old guy ( in his 70s but ex plumber ;- ) who lives a few doors away, he asked had I checked the overflow on the sink, to which I said no, he told me to disconnect the pipe and clean it as sometimes food debris gets inside and leads to smells, toom it out and sure enough it was full of gunge, cleaned out and that sorted that problem. this maybe of some help. plumbing is a minefield if ya like me and know not very much at all about it 
