It turned out is was the rear tyre. I noticed the stick on reflector was detached over an area. On the rack it was easy to see the bulge. Sidewall blown out. Threw on a replacement schwalbe supreme, good as new! Guess I rebuilt my front wheel on a lazy hunch but it was in such bad shape it needed doing anyhow. Funny how it really seemed to come from the front perhaps because you feel it thru your grips.