Definitely check your tyres first. Also I'd be checking the rim/s to make sure they're still perfectly round. Although with rims the noise tends to be there all the time, obviously varying according to speed and how damaged the rim is.
Another though, although probably not so likely. If a tyre has an unusually thick spot where the join is, that can produce noises too (most would probably miss it though). To be able to pick that though, you need to find a perfect smooth surface, go very, very slowly, keeping both hands on the wheel so you get the feed back, and if there is a point where it is thicker than it should be, you should be able to feel a slight lump on the offending tyre. Try a few times to be fairly sure.
Had a "thick tyre" 2 sets back, and when we took the tyre off the rim, it was quite noticeable. Tyre shop were very good as the tyres hadn't done much mileage, and swapped them over for a totally new set. I've spent a lot of money with that tyre shop and send them a lot of business, so that may have some bearing.