I had aftermarket rims installed on CTS and the weight were mounted on the inside of the rim.Now I notice a very slight vibration between 50-60.

I had aftermarket rims installed on CTS and the weight were mounted on the inside of the rim.Now I notice a very slight vibration between 50-60.
Most aftermarket rims use stick on weights inside the wheel. These wheels are typically flangeless, as most facotry wheels have a lip around the wheel were weights are added. These stick on weights can fall off. Sounds like you need to get them rebalanced.

Weights should be placed on the inside for magnesium and aluminum wheels. You can't actually get a true balance with lip mounted weights. Any tire place worth anything will know this.

Generally, tire places will only use the lip weights on stock steel rims on low performance vehicles.

There's been a lot of discussion on wheel vibration. This is not a unique problem with the CTS but it is probably going to be a recurring one because the car rides so smooth and it's very noticeable when it happens.

Here's some links if you want to read more from other sources:

http://www.clubcobra.com/t37899.html

This one is interesting, it actually talks about the use of a locking nut being enough imbalance to cause vibration:

http://www.miata.net/garage/65_mph.html

One from Hunter:

http://128.242.141.111/pub/features/intro.cfm

stick on weights on the inside of the wheel only, indicate that the wheel has only been static balanced. to properly balance a wheel without weights on the outside there should be stick on weights in the center of the wheel as well as standard clamp-on weights on the inside flange. this ensures that the wheel has been properly statically balanced as well as dynamically balanced. static balance only makes up for the "up and down" variance in the wheel. dynamic balance makes up for the "side to side" variations. a wheel canot be properly balanced if it is not balanced in this manner.

stick on weights on the inside of the wheel only, indicate that the wheel has only been static balanced. to properly balance a wheel without weights on the outside there should be stick on weights in the center of the wheel as well as standard clamp-on weights on the inside flange. this ensures that the wheel has been properly statically balanced as well as dynamically balanced. static balance only makes up for the "up and down" variance in the wheel. dynamic balance makes up for the "side to side" variations. a wheel canot be properly balanced if it is not balanced in this manner.
Most aftermarket wheels these days do not have a flange to clamp any weights on. They normally stick on weights on the inside or outside of the inner wheel (not the center) to accomplish what you are talking about.

Got your hub centrics installed??????