Is it normal that i can hum the beat to a song i never heard before?

The beat.. relatively simple, it's repetitive, you only need to listen to a few seconds memorize the loop And keep repeating it. Words? One words can only logically follow another in a specific context , so If I go: got a crush on...
how do you assume the l sentence is going to end?
On a girl or something..
you'd never assume it's... a pretty pistol.
Shoould I tell that I feel... how many ways are there to complete this sentence? which one is the most obvious?
So you basically get the "theme" of the song, and your brain automatically processes it, goes in search of the the other words you need to complete the conceptual/meaning logical sequence.. since most songs today haven't got very complex meaning/ Linguistic scheme lyrics, you end up predicting how a sentence is going to continue and eventually end.. and which sentence/word is likely to follow. Try doing it with a completely different music, possibly classical music.
I do the same thing with classical music. Very complex patterns of sounds.. like: what is the most logical harmonic sequence of notes that could be following this one I just listened to? And my brain goes: la la la la la la.
Automatically. This is just one of the few ways this phenomenon can take place. The other one is delayed processing of auditive input, basically, you think you are predicting the words because it takes a few milliseconds for the sound you are hearing to get processes, so in reality, in that case, you are just singing along with a slight delay. A delay so short, it might actually seem to you and other people around you, you are actually predicting, anticipating the words. But you are merely singing along in delay.

How do you know you're humming the beat to certain song when you've never even heard said song before?

All Metallica songs are imprinted into my brain.

Doing this to most popular forms of music is easy because they follow relatively simple patterns.

And then there are eventual errors in hearing the sounds of which the song is made out of, especially if you are slightly deaf due to exposure to blasting music or the volume of it is high enough to cause distortions, so a word might sound like another one,but only to you.
It is kind of different when all of a sudden a completely different sentence comes out of your speakers in a totally different frequence, it's clear somebody planted it there.
It is definitely a cruel prank to play on a manic-depressive or a schizophrenic. I am talking too much again. Tudiju!

Anyone could do it, all u need to hear one bar and it repeats over and over

That's pretty cool! Normal? I think so.