Mmm this could be the BB.
I suggest that u drain the oil from the BB. Then filter it though a mesh filter ..or a pair of ladies tights.
See if there are any metal filings or chips of BB in the residue .
I suspect that some BB gear mesh can break off/ fail. Causing the Bang which is the slack taking up.
Anyway it's no big deal to do this ....if the oil if clear ...then u know it's all ok internally.
Hope this helps.
