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try gentle braking when you hear the noise.
try to emulate a situation you always get the noise and do gentle braking.
if the sound disappears you have a loose anti-rattle plate behind your brake pad.
they are mild steel and bend easily so don't retain themselves in position.

First thing is to determine whether it's road speed related, engine speed related or not related to either.

If it's road speed and it's at the front then you need to start looking at front hubs and brakes. Jack it up and feel for looseness in the wheel bearings or things like pads catching on the slots in the brake rotors (if you have slots), etc. Also check tyres for damage and big nails or rocks embedded in them.

If it's engine speed related then you can try and hunt it down by touch and ear. A mechanics stethoscope is handy, otherwise make one with plastic tubing.

If it's just random and fairly light tapping then it's most likely some loose bodywork or something bolted to the body rattling. I tracked down a similar noise last week - it was a bolt holding the back of the headlight retractor assembly to the body. If it's a pretty meaty sound then loose suspension components or worn control arm bushes are possibles. Either way you just have to try prying and moving everything you can until you find it.

Shock mounts or shock shaft nut

I had similar few years back, the screw holding the clamp for abs line came loose. So it rattled around the shock.

Wheel nuts tight?!

Clear out behind the bottom of the quarter panels, something might have fallen in there, if not at least you'll know they're clean and not going to rust.

Indicator lamp plug hanging loose?

Check inside your doors (take the door card off from the inside) and see if there are any loose wires or plugs. I had a noise similar to this a while back and couldnt for the life of me work out what it was until I was lubing the windows one day. I think it was a connector for the window reg, the plastic clip had broken off so it was no longer secured to the door so it would bang and tap against the inside of the door or window if that was down.

Power plant frame (the long alloy C section that connects the gearbox to the rear end can crack and the long bolts at the back could be loose or missing. Jack the car up, put some stands under it and have a good look at it and run a spanner over the bolts.

Would the noise be picked up by a mic of some sort? A video of the noise under the driving conditions where it occurs might help with more suggestions.

Front or rear swaybar endlinks?

There is a tool called electronic ears, or electronic stethoscope.
It's just a fancy version of the typical mechanics stethoscope, but uses sensitive microphones.
They also exist in wireless. So you can place microphones somewhere you suspect a noise and just go for a drive with the headphone on. If you hear a really loud knock, you found your fault. If not, move microphone and try again. Only problem I can foresee is trying to secure the microphones.
Here is the wired one, go to home page and you'll find wired one. There are other cheaper ones around. This is just one I know of because I sell it to workshops.
http://www.warrenandbrown.com.au/precisiontools/product-details/chassis-ear-electronic-squeak-&-rattle-finder/269

the other great way to find things- if you know the right person- is the mechanic with great ears and intelligence. I remember selling a late model car with pretty high kms and I knew some of its problems. Nothing hidden and nothing to hide. A buyer turns up and takes it for a drive and picks every fault and one or two extras- he was a mechanic- he was happy to buy the car and did. He was one of those guys like some musicians that can hear the sound that mere mortals don't. For those of us with limited experience and not working on cars for a living how do you know what certain things sound like and how on earth do you describe something with any accuracy to someone else.

Strut top nut, swaybar end links, strut mounting nuts

It's clearly apparent those bus don't look healthy.