Strange "Growl" sound from 99' 540i

My car started to make this odd growl sound after I replaced the water pump, the serpentine belt and the water pump pulley the other day. I have a feeling the sound does not coincide with the maintenance done. It seems to be coming from the back of the engine. Any ideas? The car had been sitting a few weeks until I got around to fixing it and it was not making this sound prior to starting it back yesterday. I searched around for a similar sound and wasn't able to find one. Also there are no check engine lights on or other warnings. Here is a video with the sound:

Might be a sludged tappet. How often do you change oil?

Maybe it's pissed because it has to wear those groady-looking valve covers!

Lol I agree the valve covers look like crap, but it was bought that way. I have been more focused on the mechanical issues rather than aesthetics for now. I change the oil about every 3k miles and it's due for another one in about 1k. Currently has 94k on it.

Scavenging PS Pump... was your wheel turned in either direction? Did you check fluid level?

PS fluid is fine, also the wheels were straight when the video was taken. The sound seems to be somewhat constant. I can hear it while I drive ( I took it for a 50 mile drive today just to ensure everything was working ok after the water pump replacement). The noise always seemed to be there sometimes it would quiet down though. Could this possibly be a sign of my Secondary air pump about to fail? Or perhaps a manifold leak?