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Sounds like oil not getting to one of the lifters at idle, something in the upper drive train. When rpm is raised making the noise seem to go away. The extra oil pressure dampens the slap sound.
My e34 made this sound every winter. I used esso xd 30 diesel oil one summer to clean out the engine and didn't make any noise the following winter.
Sounds like oil not getting to one of the lifters at idle
This. Could be a bad lifter, clogged passage or a dying oil pump. Could be worth trying different grades of oil, not sure which though. A thinner oil would get through clogs easier but thicker oil would make it easier to maintain higher pressure and stay in the lifter easier.
Get a long screwdriver (> 12") place the handle on your ear, and the tip on the valve cover, water pump and other areas. You will be amazed at what you can hear. This will allow you to pin point where the problem is coming from.
Definitely not a lifter. Sounds more like a noisy injector to me. Hard to tell on the video though, it also kinda sounds like an electrical arcing a little too.
First thing to do is to take off the plastic cover, as well as the ones over the coils. Those really dampen sounds and make it hard to pinpoint locations. Then as Max_vq suggested use a screwdriver handle or a stethoscope.
I've had a loose plug on mine, and it didn't sound like that because the plastic cover over the coils muffled it. But ya never know, so it's worth a check. You'll know very quickly just by pulling the plastic and having a listen.
seriously I get the same noise but only when I track it (hard driving) as soon as I get back to normal driving it stops I've talked about this problem to Dr Vanos and they are almost certain it's a lifter not holding it's oil. (my car 1997 M3) mine are hydrolic lifters so I was told that under harsh driving they discharge and thats why I hear the rattle noises. As driving gets normal they build back up and rattle stops. Is this something you hear all the time? if so then you most have mechanical lifter which would rattle all the time.
if it's the lifters then it's no dangerous but very annoying.