Soundpost issues?

Check nut slots (tiny piece of card in the slot under the string and bring to pitch just eliminate that possibility)
Check loose tuner gears?
Check bridge foot (if not fitting evenly it can buzz)

Other than that, you might just have to pay to have it looked at ...

 

check for an open seam

 

+1, my first thought too.

 

+2

 

i just fixed a similar issue by cutting the endpin down

mine was rattling on open strings too, i just pulled the endpin out to where the rattle stopped and cut it off that much
hope your is as simple

 

There are so many things this could be, I pity you, it's so annoying. Been there several times myself, it can be a complete mystery.
What makes it worse is that the acoustics of the bass are perfect for fooling you about where the sound is coming from. It might be loudest by the f-hole, but don't be fooled.
By way of solidarity, here's my checklist:

Bridge: Are the feet perfectly flat on the front?
Strings: Is it a loose winding? If the buzz is just on one string, and buzzes wherever you stop the string, it could be. But if it's only on one note, and buzzes if you play that note on a different string, then it'll be on the bass itself.
String ends in peg-box: Doesn't sound like your problem, but always worth checking. Are they rubbing against the sides?
Tail wire: Is there a loose end vibrating against the bass?
End-pin: Try it futher in / out, really tighten the screw. Put a bit of tape around the pin to tighten it in the hole.
Tuners / tuner plates: Give every screw a quick tighten, especially the ones holding the actual machine heads on.
Nut slots: Are they too big? Like previous poster said, pad it with paper strip as an experiment.
Pickup wires: You got any cables rubbing against body?

Still no idea: Lie bass flat, loosen strings, adjust and reposition bridge, adjust tailwire and tailpiece, re-tension strings, make sure bridge stays upright and feet flat. Has it gone yet?

If you still can't find it, you have to hope it's not a loose bass bar. Or a loose seam. You can try putting pressure on different parts of the bass to see if the buzz stops. Easier said than done, and even then it can be misleading, as the pressure may just reduce overall resonance enough to cut out the buzz, even if you're pressing on the wrong bit.
But if you find a spot where even gentle pressure can switch the buzz in and out, you might have found it.

Good luck.

 

Tap with a knuckle around the perimeter of the top and back where the ribs meet. If you have an open seam, the sound changes from thunk to clack at the area of the open seam.

 

I'd repost in the setup and repair section, but since you're here: Doubtful if it's the soundpost. It's either up or down and rarely buzzes. Could be a bad string - loose winding or something. Does it buzz more on a certain note? Could be a loose seam or other place on the structure of the bass. Loose pickup, if you have one on, or other piece touching one of the strings, check the afterlength. could be a tuner or screw or something else on the scroll that's rattling. Buzzes drive me batty and those are some of the usual suspects. -ike

 

braydensharrar -- sounds like something i struggled with for quite a while since Jan 08. on an open A string, i could hear a faint vibration "buzz" sound that i couldn't locate for the longest time. it finally became clear: it was the bridge foot on the E side closest to the ff hole that was making the sound, rattling against the top. but get this -- the real cause of that was the sound post being placed too far out on the other side of the bass top! i finally figured this out last month, June 09, about 18 months later.

here's a link to the thread i started to try and find some advice. http://www.talkbass.com/forum/showthread.php?t=442514

funny, at first i didn't think the sound post was a realistic possible cause, even though that was mentioned in the laundry list of things to check. but that is in fact what it turned out to be, much to the amazement of some pretty experienced luthiers who helped me here on TB. maybe my experience will help you find out what your issue really is. good luck...

 

I had the same thing - a rattling that sounded like it was in the body, but my teacher assured me it was the endpin and getting this replaced, cured the problem.

It's easy to check as you can just try playing without the endpin extended and see if this affects the rattle?

 

ike -- right, how can a sound post buzz? it can't because it is under pressure. but if the SP is not supporting the top in a place symmetrical to the bass bar and bridge feet, like way out in the SE Territory, the E bridge foot can rattle a bit and create a faint buzz! it happened to me. sometimes the bass is a mysterious instrument!