Thump/knock engine bay sound

Christian87n

Lurker
Feb 19, 2018
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2012 335is
I heard this light thumping/knocking sound by my inlets/exhaust side while doing a routine check for leaks. It appears that it’s not rpm dependent. In the video, I took it from idle to 1000rpm, 1500, and 2,000 and the tempo of the sound did not change. It only changed and went away after I shut the engine down. It’s light in volume and unless you’re really looking for problems, you wouldn’t normally hear it. I put my phone by the front inlet to be able to record the sound. Any ideas??

 

Christian87n

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Feb 19, 2018
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2012 335is
Sounds like gurgling fluid. Coolant maybe?

Water pump crossed my mind for that reason but I’ve never heard a water pump sound like that before. Also, my coolant tank was relocated with the 335d tank, so it’s now on the other side of the engine bay.
 

Christian87n

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Feb 19, 2018
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hmm. I guess I’ll buy one soon in case it decides to quit, but I haven’t had any cooling issues. Thanks!
 

doublespaces

General of the Army
Oct 18, 2016
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If it is the water pump, just pull the plug on it and see if it still happens or manually engage the water pump bleeding process.
 

Christian87n

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Feb 19, 2018
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So I just ran the water pump bleed procedure. I didn’t get the thumping sound at all. While I’m at it though, I’ve never heard what a normal pump should sound like, so here’s a video of it running during the first few minutes of the procedure.


I also started the engine immediately after and managed to record the thumping sound again. It placed my hand around and can’t feel any thumping. It even keeps going for a second or two after engine shutdown. Sounds like someone tapping a pvc pipe.

 

Erichale77

Lieutenant
Nov 14, 2016
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Clearwater, FL
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Still sounds like fluid gurgling in a pipe. Perhaps water pump impeller is broke and antifreeze isn't circulating properly?
 

Christian87n

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Feb 19, 2018
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2012 335is
So an update! It wasn’t the water pump. It appears to have been the pcv vent, although for the life of me, I can’t figure out how it got fixed by what I did.

51C72D94-3F28-43CC-B7DB-C0895B39FA0D.jpeg


I managed to isolate the sound to the front inlet and worked my problem from there. I took off the filter and it got louder. As soon as I put my hand to plug the hole in the inlet for the hose, the noise stopped.

Following the hose circled in red, I disconnected it from the heating element and found the source of the sound there. I also found the wire disconnected from the heating element, so I reinstalled it. Figuring it was probably the flapper/check valve, I reinstalled everything back and when I turned on the car, the noise never came back.

My understanding was the wire that goes to the heating element was the power source only. So now I’m wondering what part of that whole assembly was causing it.

Also, I believe I was feeling suction whenever I placed my hand to block the sound. Does that flapper suck air at idle for vacuum control reasons and blow air at boost or is mine faulty?
 

Christian87n

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Feb 19, 2018
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2012 335is
Good that you fixed it. What inlets are those?

Thanks! I’m relieved it wasn’t something worse but I still wonder how the sound stopped by simply reconnecting the power back to the heating element.

The inlets are TFT inlets with the chargepipe. I’ve been very happy with them.
 

doublespaces

General of the Army
Oct 18, 2016
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What is the temperature where you live? I can't remember the exact purpose of the heater element, but its designed for very cold climates and it has to do with moisture either entering or exiting the crank case. If it wasn't plugged in, then it wouldn't be operating. Maybe the flapper valve got jammed because of this and that somehow had an impact on the crank case ventilation. Glugging oil might make a noise like that and residual pressure would explain why it would continue to happen after the engine was shutdown.

I think @[email protected] might actually have better insight into this.
 

Christian87n

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Feb 19, 2018
24
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2012 335is
Your description of the system was my understanding as well. I didn’t think it was cold enough here but I guess maybe it is. Haha.

I live in San Antonio, Texas, so the coldest we had this year was low 30s. Lately, it’s been low 60s to high 70s. At one point about a month and change ago, the oil was very milky, so I took it for a long drive to burn the moisture out.

It’s been clean since that drive and the sound hasn’t returned since I plugged the wire back.
 
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Marq_Z

New Member
Feb 19, 2020
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So an update! It wasn’t the water pump. It appears to have been the pcv vent, although for the life of me, I can’t figure out how it got fixed by what I did.

View attachment 9819

I managed to isolate the sound to the front inlet and worked my problem from there. I took off the filter and it got louder. As soon as I put my hand to plug the hole in the inlet for the hose, the noise stopped.

Following the hose circled in red, I disconnected it from the heating element and found the source of the sound there. I also found the wire disconnected from the heating element, so I reinstalled it. Figuring it was probably the flapper/check valve, I reinstalled everything back and when I turned on the car, the noise never came back.

My understanding was the wire that goes to the heating element was the power source only. So now I’m wondering what part of that whole assembly was causing it.

Also, I believe I was feeling suction whenever I placed my hand to block the sound. Does that flapper suck air at idle for vacuum control reasons and blow air at boost or is mine faulty?


Do you remember if the noise was always there from cold start? I have the same noise in my N55 f30 and I cant find the source. The thumping/knocking on my car starts after I've driven for awhile and the car is hot. Help!

Thanks.
 

Christian87n

Lurker
Feb 19, 2018
24
6
0
Ride
2012 335is
Do you remember if the noise was always there from cold start? I have the same noise in my N55 f30 and I cant find the source. The thumping/knocking on my car starts after I've driven for awhile and the car is hot. Help!

Thanks.

it was always there regardless of temp.