Last night I removed my pcv box and its hoses on my 91 740 sedan, cleaned them with brake cleaner, and replaced them. This morning i noticed that there's a crackling sound coming from my engine bay, non-rhythmic and random, almost like the crackling of a fireplace. Hopefully im able to upload the video and the sound is clear. I'm concerned i dislodged or broke something trying to get the box out and back in. Any idea what it could be?

Noise: https://youtu.be/B3A7XMdSowo

I can definitely hear a rattling noise in this video. It sounds rhythmic like something is touching something that's rotating, but like the item being hit is moving around.

What all did you remove to do it?
Did you drop anything and not recover it?

Brake cleaner is very hard on the plastic and the sealant. I stopped cleaning my breather boxes after having to remove them and replace them a few months after cleaning them.

Here's a terrible idea - some chunk of oil residue is light enough to float inside the breather box and not escape, and it's hitting the walls now and then. That would explain why it's intermittent.

That might be what it was. I stopped noticing the sound a couple days after, - if it was a chunk of oil residue it may have gradually worn itself down and broken apart.
I didnt remove anything to get to it, which may or may not have been a good idea given how many cables i inadvertently tugged on struggling to get the box in/out. Obviously i did remove the hoses as well. I did actually remove some harness for another cable built into the top of the pcv box but replaced it after and made sure the cable it was meant for was snug.
I remember an old ziptie around some cables breaking off and not being able to find it and i thought it might be stuck somewhere and being blown around by the radiator but i later found it in the driver side footwell for some inexplicable reason. Though none of this would explain why the noise occured at startup, went away once the engine had warmed up, then reoccurred at startup another day.

Hi,

I’m sorry but I cannot separate a crackling noise from all the other regular noises.

You seem to move the camera around from one side to the other so I’m not sure you can decide where the sound jumps out from.
I hear what sounds like a brief puff of wind but that could be from the fan.
Cracklings from a fireplace is usually water or other gases vaporizing out out the wood.
The crankcase doesn’t usually exhibit audio sounds as there no pressure chamber to suddenly release built up pressures.

A clacking could be a water pump bearing, power steering vane or even an alternator bearing.
I would suggest a mechanics stethoscope to isolate any noise in a pump housing.
An old V belt can thump a little too, so how old are yours.
You can remove one at a time to see if something driven makes it stop and that narrows things down a bit. It will also remove unwanted noises.

Hope my suggestions help you “feel the car out’” a little better!

Phil

i should certainly look into a stethoscope. And my belts are already on my to do list so it may be related to that. Any chance excess brake cleaner in the combustion chamber could cause a crackling noise?

Hi,

Brake cleaner evaporates pretty darn readily, so, I doubt that it would still be laying about to cause any cracklings.

This follow up explanation might have clues even if its gone away for now!

If you pulled and yanked on hoses, like you said you did to get the box out, then maybe you got a leakage of coolant some place.
Those lines are just up and to the right of the flame trap box.
They are in hard place to just change out and maybe there is a rubbed spot on one of the hoses.
They can last a long long time and never seem to get changed. I’m guilty of that!
The oil dip tube can rub on a hose there.
I have tweaked mine away from that curved hose.

It’s Possible that it started dripping over to the exhaust manifold side by running underneath out of sight.
There is a metal line that goes around the back of the engine to the water pump return hose.
It’s made of steel and can rust if the coolant is not maintained properly!
You might be lucky and it sealed its self, for now, anyways!
Watch your coolant level with a dark marker line on the side of the bottle and sniff for an antifreeze smell or residual stains.

Guess we will have to hide and watch for the thread to bounce up again!

Phil