Inregular bumpy rides

[SneakLerd]

Greetings.
I am an owner of a VW Polo 2000 model, driven 110 900km.
Have had some problems with my car for a while now.
I've had warped rotors before, but changed my rotors a few months back.
My tires are completely new, no difference between winter/summer tires. Mounted in a authorized workshop.
My tires have their recommended air pressure.
The handbrake has been properly adjusted, and goes only a maximum of two clicks.

The problem I am experiencing is that sometimes I get a lot of vibration in my car.
When I drive at ~20km/h I can feel the back dipping up and down, kind of hard to explain.
The faster I drive, the higher the frequency of the vibration, but some times, there is no vibration at all.
I do not know if has something to do with it, but when I am driving slowly, and pressurizing the brake pedal. I can hear a screak from my rear, like something is releasing slowly (idk) it has done that for around a year or so.

Thank you for all of your ideas

 

[Ralston18]

I would look into tire alignments: front and rear.

Then the shock absorbers.

But please remember that even "authorized" workshops can and do make mistakes.

Take the VW back and report the problem(s).

 

[mjslakeridge]

Warped rotors will only produce a vibration when brake pressure is applied. As suggested, an alignment may be necessary. I had a vibration in my old car that increased as the speed increased, and it was bad wheel bearings. It also made quite a loud noise at certain speeds.

 

[SneakLerd]

Even though contrary belief, the problem were laying that I did not have hub center rings.
After installing those, the problem were no longer existant.

Regards