Preface: I've never messed with a headset, threaded or threadless, before and I'm figuring this out as I go along so any advice/information/links are appreciated.

Edit: So basically I've been hearing a knocking from the steerer tube area any time I hit a harsh bump and initially when I took it over to Rapid Transit a mechanic said he thought I just needed to tighten the top cap. I did that and it seemed to have gone away for a week or two. Then I rode off quite a few curbs and it started coming back but when I went to tighten it again the top cap just popped out and seems to have seized. I guess my question is whether the bike is still safe to ride if I just pound the top cap back in like I did before (no issues in the last few days other than the noise) and whether this is indicative of a more serious problem or if I just need a new top cap.

TIA,
Dan

Some stuff here to get you started... or LBS can whip it into shape pretty quick.

Star Nut (under top cap) install info
https://www.parktool.com/repair/readhowto.asp?id=67

Threadless Headset service info
https://www.parktool.com/repair/readhowto.asp?id=65

the star fangle nut has been pulled out. it is actually a separate piece from the top cap.
The star nut is supposed to be stuck in the steering tube.

you might need help from the LBS
but here is how it gets installed
https://www.parktool.com/repair/readhowto.asp?id=67

haha i didn't see that someone had already answered the question. oopsy

thanks guys!

You can look into a headlock to keep that from happening in the future to replace the star all together.

I know I've seen them somewhere else recently, but I found these...
https://www.google.com/products?q=hea...ock&hl=en&aq=f
https://www.setteusa.com/show.php/Parts|Spacers|headlock
https://www.pricepoint.com/detail/162...0-Headlock.htm

This is kinda neat to replace spacers if you have any, holds the fork in when you adjust the stem
https://store.interlocracing.com/opslohesp.html