Brakes make a screech noise, not consistent or as loud as other times

A few months back I decided to do some maintenance on my girlfriend's car listed above S model. I did as much work as I could with what I had, with the help of her cousin (who normally does do the maintenance on the car to begin with), I knew absolutely that the shocks and struts needed to be replaced along with the oil, spark plugs, PCV valve, brakes, and rotors. Bought everything brand new we had no hiccups, errors, nothing, yet even though we have done all that the brakes are still making a bad screeching sound and it gets worse the longer it’s driven. Her cousin and I both took each tire off and checked all of the brakes for any type of bad wearing of the brake pads, which looks like they are all still almost brand new, we looked for an indication of inconsistencies of the wearing rotors which none can be found, we looked for dirt or dust on the brake pads and the rotors there’s nothing, there is no indication of anything wrong with the brakes except for normal wear and tear on the brakes, so both him and I are at a loss, so please is there anyone that can help me out?