My car's making a strange sound.?

I have a ten year old mondeo, and for the last three days a strange noise has been coming from the engine [I think]. It sounds like I'm traveling at 40 mph but in third gear. It's a constant noise that's just slightly louder than the normal engine noise that I can hear underneath it. It doesn't change pitch in any way when I change gear, but it does increase in volume if I go faster, but only in a subtle way. It doesn't seem to affect the efficiency of the car as such, but I haven't pushed it over 45 mph as it doesn't feel right. I've booked it into the garage for Monday, but in the meantime if anybody has any ideas I'd be interested to hear them. Oh, we had a new tyre put on a week ago, and a friend suggested it's the tyre noise, but I've never heard of that being audible to the driver.