Could be the onset of big end knock from the symptoms you describe. This is most prevalent when the engine is under load and the bearings have to absorb piston thrust i.e. letting the clutch out. When just ticking over in neutral it may not be all that apparent, or when cold as the oil is at its thickest and oil pressure is higher than when hot. Also you may not notice slight big end knock at speed due to all the usual knocks and rattles, wind noise etc.
If you suspect big end wear, FFS get it stripped and inspected before the crankshaft journals get worn. If you catch it early enough you can get away with changing the big end shells without a journal regrind / crankshaft replacement. I would also replace the mains as well. Much cheaper than a new engine if it lunches itself.
Either way, if it is definitely knocking, and with 150k on the odometer, it is worth a precautionary strip and inspect. Not sure if this can be done in-situ or if the engine needs removal, all my experience is in big marine diesels and some automotive work, I have no hands-on experience with the BWM boxer twin internals.
