Sorry - I don't know anything about Scoobynet (i.e. whether it's for owners of performance cars or ordinary cars) but I have a weird problem with my Renault Safrane and I wondered if anyone here could help?

I have a 3.0 litre V6 Renauilt Safrane automatic. It's about 11 years old but I only do about 8,000 miles a year so it doesn't get hammered. A year ago I heard a very low frequency sound coming from underneath the car. The sound was somewhere between a rumble and a buzz but very low frequency with no high frequency content. It turned out that either the silencer or catalytic converter had blown so I had them both replaced. Everything's been fine since then.

Now, this sound has come back again. It's at a much lower level than before. In fact, if you were a passenger in the car you wouldn't even notice it around town. But once on the motorway, it's very noticeable. Today, I took the car back to have the silencer checked but they told me that everything's okay. They actually let me stand underneath the car to look and listen but unfortunately, the sound doesn't occur at low speeds or when the car is idling. When I hear it though, it sounds very 'rough' - as though the car isn't firing on all cylinders. However, the car's performance isn't affected in any way. It still accelerates okay and there aren't any knocking noises or difficulty when I'm starting it up. I thought that maybe the wheels might need balancing but there's no juddering or anything. Also, the oil level & pressure seem fine and I don't get any warning lights coming on. For all the world, it honestly does sound like a hole in the exhaust but the mechanics tell me there isn't one.

Does this ring any bells with anyone here? What else could it be if the silencer's okay?

[Edit...]

Sorry guys - I just realised that this forum is for Subaru owners - but if anyone can anyone suggest somewhere more appropriate, I'd be very grateful. Thanks....