So, it happened after an oil change and there's hot weather. I'd say pull out that oil ASAP. Always think "simple". If you change one thing and something breaks, then, it's probably the thing that you just changed, not something exotic and different. Run some Mobil 1 Synthetic 15w50, and my bet is that it'll fix everything. I know, I know, BMW now says something different due to their contract with Castrol or something of the sort... but just run the Mobil 1 15w50 now, and see what happens.
Oil viscosity being the problem does not intuitively make any sense in the situation, except that it's what you just changed. Also, barring a better description, I'm going to blame low oil viscosity and an aging VANOS unit. It's a real stretch, but it's a cheap attempt, and at least it'd take it off the table.
Because you mentioned in your first post that it was intermittent and at 3k rpms, not 4k, and that it is "knocking", not "tapping", I don't really like my solution idea and I agree with people thinking that it's pre-ignition. I'm assuming you're throwing no CEL, though?
As for piston slap, or worn crank bearings, etc, I think that it'd be less intermittent (more constant), and pretty unlikely for a BMW engine with reasonable miles on it.
My next step after the oil change would be to look for a misfire, and probably just change the cheap ignition components, starting with plugs, cap, and rotor.
Or, maybe you just have a bad knock sensor. That's supposed to throw a CEL if that happens, I believe. Because, if the engine is actually pre-igniting, it's supposed to dump in more fuel to keep that from happening. Hmm...
I'm pretty late to the party here, so hopefully you already have it fixed. You'll have to let us know!