I have owned my car since new (99 S70GLT), and there has always been a strong knocking sound from under the hood, sounds like it's coming from the radiator area, not really the engine. I have never figured out what this is, apparently it's normal, since the car has been dealer serviced and they never mentioned it. Can someone tell me where that sound comes from? Thanks.

 

When does it happen?
When running?
When driving on flat road?
When driving over a bumpy road?

 

I hear it whenever the engine is running. Sounds like the air pump I use to fill up my tires but not as fast or as loud. Maybe about 180 knocks per minute. Which just made me think, maybe it's the air pump, does that have a steady knocking sound?

 

No the air pump does not.
Maybe lifters making nosie?

 

i have the same thing, but it sounds like its coming from under the car. it gets louder when the car is in D. and its more of a rattle than a knock. i might post a video soon because i am concerned about it... however i don't want to know what volvo is going to tell me if i bring it there

 

mine does that too. i am assuming its normal (mines not obnoxiously loud), so im not worried about it.

 

Apparently it's a '99 problem. My '98 doesn't have any sort of noise like that...just a constant CEL because I need to get rid of my generic gas cap and buy a Volvo gas cap.