Noisey frame amd other newbie annoyances

When you say slipping out of gear, do you mean the chain is literally slipping off the sprocket, or is it momentarily dropping between sprockets?  If so, it could be that the rear derailleur sis not indexed properly- try tweaking the alignment of the rear derailleur ideally with the bike on a stand, and see if that improves it.

Unless the chain length is miles out, the rear derailleur spring should keep enough tension on it.

Another culprit could be the freehub not engaging as well as it might.

Re the rattle I would echo the loose headset is a possibility... to test, place the bike on firm ground with the front brake on, push it back and forth... you will feel any loseness in the headset, and this can manifest in a nasty rattle on rougher ground.

Bottle cage bolts come loose quite often, and sound like your entire bike is falling apart. Also, saddle/ seatpost squeaks and rattles can sound very much like frame noises.

1. Check for frame cracks at weld joints

2. Pick up the bike and shake it.  Listen for cables slapping the inside walls of the frame. Try to pinpoint the "rattle" and if its metal and against metal, or....

3. Check if your headset is loose. Lock the front brake and move the bike and forth. It should not have any play.

4. Check for loose cassette lock ring. With your fingers move the cassette from side to side. It sould not have any play.

Report back

For chain length sizing see this https://www.parktool.com/blog/repair-help/chain-length-sizing

For rough roads I go one less chain link. This helped my chain from bouncing off the big to small ring.