Bump coming from the back
I recently purchased a used, rigid frame MTB. I took a ride on it before I bought it, and everything seemed fine. As I will be doing most of my riding in the city, I bought a pair of slick tires.
With the narrower tires, I now notice something I did not before. There is a very slight, consistent (and evenly spaced) bump, coming from what seems like the back wheel, or at least the rear of the bike. It's like I am riding over very tiny speed bumps, and only my back wheel is hitting them. Again, it is very slight, I only noticed it with the new skinny tires.
As to what might be causing it, I am very inexperienced with bicycle repair (but want to learn!) but I have managed check a few things. When I was changing the tires, I looked carefully at the back hub and it seemed fine. The front wheel had some spoke problems, but after my very layman observation on the back hub it seems true to me.
I would greatly appreciate any advice anyone could give on a likely cause of this problem. Thanks very much in advance!