2004 Nissan Micra Lately Car Been Making Strange Noise Exceeding 35 40 Mph

Lately my car has been making a strange noise when exceeding 35/40 mph. At first the noise was quite mild but lately it seems to have worsened and will start as i approach 30 mph. The best way to describe the noise is that it almost sounds like I need to change gear, but once the gear is changed the noise still continues. The car seems to vibrate a bit too.
Any ideas what this may be?
Thanks.