Need help diagnosing this knock-like noise coming from my '99 C280. The car has 67,000 miles and starts, runs, and drives beautifully. I just recently noticed this noise coming from the engine. It sounds like a knock and I need help diagnosing it.

Here's the video:
W202 C280 (V6) Engine Noise - YouTube

I can't pinpoint where the knocking noise is coming from. At the end of the video, there's a single loud knock/pop that made me flinch a bit. I wasn't expecting that and never heard that before.

Sounds like it could be the water pump or a pulley/tensioner on the front of the motor. The tensioner is a common issue on these.

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can you hear this tapping even when above idle speed?

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The car was parked and idling in the video, so yes.

I'm going out of town tomorrow and won't return until Monday. I'll try to take the belt off and try to isolate the cause then.

Coolant Pulley play...
W202 coolant pulley play - YouTube

I took the belt off and checked the tensioner. It spins with a ticking noise... not sure if that's normal. I also checked the coolant pulley for play, and as you can see in the above video, there is some play that causes a knocking sound identical to the video with the engine running. Replacing water pump should do it? Or should I replace the tensioner while I'm at it as well?

Tensioner video:
Nov 19, 2013 - YouTube

Thanks guys!