I have a '98 jetta and a large metal bar fell off the bottom of my car when i went over a speed bump

It's a relay that has probably gone bad

The rattle (noticed at times under hard acceleration) is the cause of gas shock on the upper part of the engine loosing charge, in-turn causing the engine mounts extra stress and thus metal vibrating/rattling together.
The knocking (over bumps, etc. if you have noted that) is caused by worn bushings, most likely worn sway bar end links (you will see the links looking at the undercarriage, on both sides, they are a little steel "bars" with threaded ends on a 90 deg angle held by bushings e.g. example of front sway bar links ).
I noticed these noises coming on (at various times) at about ~140,000km or ~67,000mi and drove the car as it was until ~300,000km when i sold it (as is). Annoying noises.... yes... did I turn up the radio and carry on driving this car, not "fixing" anything (and I mean not a single $ spent on it aside from maintenance) and then sell the car... yes. The choice is yours.

it was an exhaust hanger bracket, thro it in the trunk and one day when the flex pipe rotts thru get the nice guy who is fixing the exhaust to weld it back on!!!!

it has a metal plate on the bottom of the battery holding it onto the baseplate.long socket extension bar required to undo it.

check the frunt break pads if the pads were ever replaced by aftermarket pads they will make a clunk sound going over bumps

where is the idle speed control valve on my 98 2.0 volks. jetta

Check the ball joint, tie rods ( inner and outer ), sway bar end link and the strut for a broken spring or strut mount/bearing. It could also be something as simple as the battery being loose. I hope this was helpful for you.

Bryon

Sorry I see that it is a Jetta, the window has 2 tabs that are glued to the bottom of the glass these will likely need to be rebonded.

the same thing happened to my 1996 jetta trek.

when the alarm went off on me and the car would not start i walked around to the passenger door, locked and unlocked it, therefore the alarm went off.

but the alarm only went off if the car battery was low and had trouble starting.

Take a close look at your springs. My 99 Audi A4 had a creaking rattle that happened when I went over a speed bump or dip in the road, and it turned out the bottom of each rear spring was broken. I replaced them and the noise went away.

Another possibility would be the sway bar end links. Since the sway bar is worked pretty hard, the bushings in the end links will wear out eventually. This can cause some clanking. Look behind the wheel hubs - you should see a short bar or bracket attached to the hub at one end, and to a bar at the other end. The bar will go forward or back a short way, and then across the bottom of the car to the other wheel hub. That's your sway bar. The bushings in the end links (the short attachment bars that anchor it to the wheel hub) or in the body brackets (some cars bold the center of the sway bar to the body with a bushing in a bracket) could be deteriorated and allowing the bar to bang around on the body.

Finally, take a look at the suspension itself. Do any of the bushings where various parts meet appear to be cracked, leaking fluid, or gone entirely? There should be no gaps between any suspension pieces at the points where they meet. If there are, you're missing some bushings and may well have found the source of your noise.