Cayenne Vibration
I didn't see anyone mention this from a search query so maybe it's just my vehicle.
My S is brand new, only about 400 miles w/ the standard 18" M & S Conti's. Today, for the first time, my wife drove me to try the vehicle out for herself (she has a X5). To be honest, I had noticed that the vehicle vibrated every now and then but I didn't think much of it. Maybe I was just trying to convince myself it wasn't there. However, tonight when my wife drove me, I was stunned by the amount of vibration there was on the highway while in the passenger seat. I can only conclude that since I wasn't concentrating on driving the vibration just became more apparent. When I questioned my wife, she too said that she felt the vibration every now and then. Anyhow, I'd say we going about 75mph and the road was fairly smooth. What's more, the vibration is inconsistent. Meaning, it comes and goes without any real reason. Sometimes it's there and sometimes it's not. Looking back, I know that I've experienced it while driving as low as 50-mph +/-.
One other thing, at lower speeds like about 40-45 mph or so, I occasionally get a very loud low frequency hum in the cabin. Any thoughts? BTW, I'm use to a loud cabin too. I've owned a Range Rover, two Discovery's and a D-90 so I know road/tire noise and that's not what I'm talking about.