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.