I did a 300 mile round trip this week and started to hear a rumbling noise on the motorway at specific speeds. Basically this happens as I get to 58mph on the speedo, then goes away when I accelerate faster, but comes back at 68mph. Again it disappears, or at least is very much reduced above this at least up to 75mph. It is not engine related as the noise is the same if I accelerate, decelerate or just coast in neutral, uphill or downhill and not affected by bends in the road. I can't tell where the noise is coming from exactly because I am driving and can't crawl around at the same time, but I'll ask my passenger to do this next time I have one. Apart from that there is no change in the car handling or fuel consumption.
I just got the timing belt done last week so thought maybe they didn't tighten up the under engine tray or something and that this was causing vibration - but I checked this and it all looks ok. I got new front tyres about a month ago as well so I thought maybe its balancing but the balancing weights and pressures look to be ok.
Its weird that it comes and goes at exact speeds. Is there anything mechanical I can check before taking it in to a mechanic, bearing in mind I don't have a ramp but can take off wheels and crawl around the car if needed.
thanks in advance for any iseas.