Bicycle Mechanics - 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!

It sounds like you have a flat spot on the rim where someone bottomed out on a curb or something. I have a rim like that too where I bottomed it out on a concrete ledge.
You may be able to find it by inspection or even a loose spoke or two where the rim is dented(like mine was).

yes, i took off the tire and on inspection that's it exactly. Any tips on how i might even it out?

yes, i took off the tire and on inspection that's it exactly. Any tips on how i might even it out?

There is a tool called a "rim puller". You need to find a shop that has one, or get one yourself. A rim puller looks like a half moon (or a dishing measurer). The inside of the crescent sits on the outside of the rim (wheels off) and there is a "hook" that pulls your rim out from the inside as you screw a large wingnut. If you do it yourself, you will need to loosen spokes around that flat spot, before you start, then pull the rim back to round (or as close as you can get to round) and then re-tighten the spokes and true the wheel. You may find after doing this that spokes that were radiating perpendicular to those that went to the flat spot are not a bit looser. tighten/loosen/true all spots that seem appropriate to make your wheel true AND round again.

If you've never trued a wheel before, then probably just take it to a bike shop and ask for a quote as to what will be cheaper, a fixit job for the rim or a whole new wheel.

Here's the goodie itself, not easy to find though :( If you get your hands on one, take things gradually.