Jocko is talking about the exhaust, yes - get hold of the tailpipe and bounce it about - see if you can hear anything.
It's always worth doing the droplink, it's so easy to rule out the most common cause. Who did the droplink, did you have a chat with them about the problem?
Are there any clues as to the area the noise is coming from? Any particular corner, or does it sound like it's all around you - coming from the bodyshell? Does steering position affect it, is it more likely to knock steering straight ahead, or on bends?
Did the noise come on gradually, or suddenly? Did it coincide with any work carried out, such as when (or soon after) you had the gearbox bearings replaced? It could be as simple as something loose, engine or gearbox mount, brake caliper carrier, subframe mountings, etc.. Lots of stuff has to be disturbed to remove the gearbox.
I've had so many different causes of knocks over the years, but metallic sounding ones are more likely to be a droplink, balljoint or top mount. Bushes usually have obvious tears or other visible damage to make that sort of noise. A loose top shock absorber nut sounds similar to a droplink. Failed engine/gearbox mounts can make some horrible noises. There are even lots of silly things on the bodywork that can rattle and knock on bumps. The most obscure one I had was a shock absorber, felt perfectly normal but knocked like a droplink when driving. After going through everything else, replacing the shock absorber cured it.
It's just a matter of working through the most likely candidates, then moving onto more obscure stuff. If someone looks it over, mention that the gearbox has been out... it's a very long shot, but if they forgot to torque something up, it could have worked loose. I've even seen a new droplink knocking after a day, because the nuts had been tightened up with the suspension off the floor - the top nut subsequently worked loose.