Neck Buzz

I seek counsel from the superior set up wizards.

After 5 years I decided to change strings from D'addario Chromas 45 - 100 to Thomastik Infeld Flats
.043 - .100, this may be relevant. Simple process but with the bass being so old it ended up being a massive set up job involving everything from soldering wires to neck adjustments etc. So the bass now works great, every note rings out clear and no dead spots have been found, for the most part I'd say it's the best shape the bass has been in since I owned it. Only I've now found a weird problem that is bugging me...

If I played a 10th of B on the E string (7th fret) and D on the G String (7th fret) there is a really bad buzz that happens and the notes don't ring out. Individually the notes ring out clear as ever but the moment you even press on the B without actually playing any note the buzz it happens. This is literally the only point on the neck this happens and I can't quite source a solution to the problem.

Any thoughts?