Downloading rap beats is a common way to test your skills as a singer or rapper. A lot of producers offer free beats that you can legally download. The most commonly available are rap beats, hip hop beats and instrumentals.
As long as you understand the copyright laws and follow the rules of the copyright holder this is very normal.
There are very few if any producers on SoundClick or anywhere that will let you download beats with no strings attached. Usually the free beats are limited to non-profit use only.
One site for rap beats that you can visit and download free rap beats is HitGrinderz.com