Help! Weird rattling noise from the engine bay. Car runs just fine

[socalr53]

2009 /E70/X5/ 4.8i with around 110k miles

This has been bothering me for almost an year. Bought a Mechanics Stethoscope from HFT , tried to probe where the noise is coming from. It sounded like it was coming from or near the thermostat, so I went ahead and replaced the thermostat. Replaced water pump also while I was there. Car was due for a coolant flush anyway. But that didn't fix the noise.

https://www.youtube.com/shorts/HUic0eX1ZAo

Please listen to the YouTube short and feel free to chime in. All kinds of guesses are welcome too!

[jbash87]

Pull the accessory belts off one-by-one and see if the noise goes away. Could be a pulley, but it also kinda sounds like the fan is hitting the shroud. Are there any loose pieces floating around in the fan shroud?

[DialedIn]

Kinda sounds like a fan blade hitting something?

[Jazzyj13]

Belt tensioner may be going

[socalr53]

Originally Posted by jbash87

Pull the accessory belts off one-by-one and see if the noise goes away. Could be a pulley, but it also kinda sounds like the fan is hitting the shroud. Are there any loose pieces floating around in the fan shroud?

Thanks for the suggestion. I did pull the accessory belt before replacing the water pump. However, that was when the engine was cold. This rattling sound comes on when the engine is at normal operating temperature.

However, good point though. I will try it one more time after a 30 minutes drive, so the engine is at normal op temp and the sound comes on on idea also

btw, no loose items near the fan and also its absolutely silent on cold start. I forgot to mention that

[socalr53]

Originally Posted by DialedIn

Kinda sounds like a fan blade hitting something?

no, nothing hits fan. checked that. Thanks for the suggestion!

[socalr53]

Originally Posted by Jazzyj13

Belt tensioner may be going

Thanks for the suggestion! Will try that if nothing else makes sense. I forgot to mention that it doesn't happen on cold start, but only when the engine reaches normal operating temperature, which lead me to replace the thermostat. I thought it comes on when the thermostat opens up and the coolant start to circulate.

If it is the belt tensioner, it should start the rattling on cold start also correct ?

[jbash87]

Originally Posted by socalr53

btw, no loose items near the fan and also its absolutely silent on cold start. I forgot to mention that

Hmmm. I'm still thinking fan especially since you say it's silent when cold and this only happens when operating temp is reached. The fan will only come on to cool the radiator when coolant is flowing through it.
Maybe try unplugging the fan when it's making the noise?

[Jazzyj13]

Originally Posted by socalr53

Thanks for the suggestion! Will try that if nothing else makes sense. I forgot to mention that it doesn't happen on cold start, but only when the engine reaches normal operating temperature, which lead me to replace the thermostat. I thought it comes on when the thermostat opens up and the coolant start to circulate.

If it is the belt tensioner, it should start the rattling on cold start also correct ?

Not necessarily, the bearings inside the tensioner could start talking once it warms up.

[socalr53]

Originally Posted by jbash87

Hmmm. I'm still thinking fan especially since you say it's silent when cold and this only happens when operating temp is reached. The fan will only come on to cool the radiator when coolant is flowing through it.
Maybe try unplugging the fan when it's making the noise?

That's a great suggestion! I didn't think about disconnecting the fan, but just did a visual check and the scope probing only.

Well, I went ahead and disconnected the fan electrical connector, after a 20 minutes drive today. It still makes the noise without the fan running :-(

Idled the car for about 5 minutes without fan and it was really mad when I connected it back. I think the temp went up without the fan running. Fan went into the high speed mode when it was reconnected with a super loud noise, blasting at full speed.

[socalr53]

Originally Posted by Jazzyj13

Not necessarily, the bearings inside the tensioner could start talking once it warms up.

It's definitely in my list of items to replace, but I am looking for other possible causes before doing so. Thanks!