2006 S40 - Helicopter Noise and Vibrations on Highway

[mcb88]

First post on the forum and not the most mechanically knowledgeable, so please be kind.

I bought a 2006 S40 2.4l for use as a backup commuter car. 120k miles. Previous owner had thorough maintenance records. I test drove it but wasn't able to take it past about 50 mph on the highway for more than a couple minutes. It drove fine--more NVH than I'm used to, but nothing out of the ordinary for an older car. It passed a prepurchase inspection with an independent mechanic with flying colors.

Well, I've had it a few weeks, and I'm noticing that past 55 mph on the highway, the engine gets louder and louder. It sounds like a helicopter or a jet plane; not that loud, but the same type of sound. It also vibrates a lot--not an issue at lower speeds--to the point that the car makes me tense to drive on the highway. Both the sound and the vibrations get worse the longer the car is at highway speeds, so at the end of a 25 minute run at 65 mph, it's worse than at minute 2 or 3. I've heard some people say that the 2.4l engine is just a noisy engine. Is that so? I've driven diesel Mercedes and economy 4 cylinders in the past, and both were loud, but not like this. And if it's not normal, any thoughts on what might be the culprit here?

[Cees Klumper]

I would start by inspecting the wheels / balancing / wheel bearings. The engine should seem quieter at speed, not louder due to increased tire and wind noise, 'drowning out' the engine.