I swapped strings this evening: took off the Spiro weich G and put a Perpetual mittel G on in its place.
All hints of buzzing have disappeared. Thank you all for the tips!
PS: No response back from Thomastik, almost two weeks after I emailed them. I got an initial reply acknowledging my query, but that was it.
I know this is an older thread, but how did you like the Perpetual G on top of the Spiro Weichs?