clunking noise while driving

recently, i popped my hood and realized my brake fluid reservoir cap was missing; i'm assuming it's been about a month or so that it's been like that b/c there was no need to pop the hood. i realized my brake fluid was/is low, so i will get some brake fluid tomorrow. for the time being i have capped the reservoir with a pill bottle top lol.
NOW-
after i did the temporary fix, i started the car and was about to go. i put the car in reverse, and the gears kinda grinded when i did that. i began reversing, and there was a slight clunk nose (kind of like gears are grinding) and in first i hear it, in second and third as well, not so much fourth. i also happened to be parked on a spot where mud and rocks have compiled due to my recent "rally" adventure. can anybody help me out??

perhaps this can help; short clip of me driving
https://www.youtube.com/watch?v=jOls_9QvAs4