I just bought a C320 4Matic with 90,000 miles on it. I noticed a ticking noise at idle. It gets faster as you accelerate and stays regardless of engine temperature. It sounds very much like a sticky lifter but does not go away. It is very noticeable but I would not consider it a knock so much as a tick.

Shortly after leaving the dealership (3 hour drive at 70mph) I slowed to about 35 mph. Just as I slowed the check engine light came on. I did fill up with premium (91 octane) fuel and was very careful about tightening the fuel cap. I tried starting the engine 4 times in a row as stated in the manual but the light remains on. I'm wondering if this could be related to the ticking noise.

The other thing I noticed was a considerable amount of road noise. Seems like a whirring noise from wheel bearings but I've never owned an AWD vehicle before so I'm wondering if the drivetrain is just going to be that loud.

I just started it up after it sat in 6o degree fahrenheit weather for 20 hours and no noise at all. I didn't have time to drive it but I'm assuming it will come back when the engine warms up. The electronic oil level checked just fine.