Hello all, i am fairly new to the hobby and my quadcopter without any stick input keeps drifting to either the left or the right. 

It happens randomly. I have attached a video of how it looks. When it starts going left, there is no stick input from me. 

What could be the cause? I have calibrated the sticks on my transmitter and also made sure betaflight shows 1500+/-1 when centered.

Thank you for your help. 

The video: 

Bumping the trim?
Loose prop or faulty motor(s)?
What's your I-term? If really low and maybe battery not centred?
Just guessing really.

Have you made sure you got all your end points and centers lined up on your radio and in betaflight?