Technical Reverse gear WOOMP sound

[cyberbird]

Hi,
For a year, I have observed that occasionally, when I put car in reverse gear, it can make a low pitch WOOMP sound, it is very noticeable sound, almost like some metals are grinding at high speed, idk.
When I hear this, I immediately engage clutch, disengage reverse back to neutral, and I go back in reverse and then its fine.
It feels almost like its not getting in reverse gear correctly.
Any ideas on what this might be? Or how I should troubleshoot this?

 

[cyberbird]

Another thing I noticed last time this happened: When I put it in reverse, I didn't feel that click feeling that you get when putting in gear. It felt very empty going into reverse. There was no feeling of resistance and then gearstick jumping in gear. It did drive in reverse but with sound. I couldn't get it into reverse correctly at that point. Even tried going forward a bit and back. So I ended up reversing slowly with the WOOMP sound.. It worked ok afterwards. Somethings wrong tho. Will have to bring it to mechanic soon.