i still have a problem with my v6.

under acceleration, there is a rattling sound coming from exhaust (at least from what I hear). This is a problem that persists for almost a year 
But only under acceleration! In idle, or coasting, the engine is almost silent.

The sound is, well, like one or few cylinders are much louder than rest of them. Actually, I was under impression that I have a cracked exhaust manifold gasket. But, some recent happenings made me thing:
- when installed new spark plugs, the rattle disappeared, but came back after a while
- I've done a 1600km trip (autobahn, on LPG), and after that, for couple of weeks, there was no rattle -> the car sounded great
- i usually drive on LPG, but from time to time, I drive few days on petrol, to prevent injectors from blocking -> few days ago, i have done so, and instantly noticed that engine runs slightly rougher, the rattle was back , and I've noticed some smoke from exhaust
- after this, it rattles both on LPG and petrol (it seems to rattle less after longer drive).
- some times, when removed foot from accelerator, there are small explosions in exhaust

Car is a 1996 2.0 V6 with 130.000 miles, and LPG fitted. It is serviced properly: new camcover gaskets, new sparkplugs and leads, no oil in plug wells, no air leaks. Petrol injectors haven't been cleaned (probably never).
It runs fine, accelerates fine, no dramatic rise of consumption.

What could be causing this? It's driving me nuts

sorry to hear, maybe have your compression tested on all 6 cyl's.
the i would go all through the engine bay and underneath following the exhaust system, tapping and pulling various parts to see if there is anything loose or just not tight enough..
also have you lpg system checked out.
sounds a lot of work but will bring you closer to understanding where it comes from and or why..
its never as simple as sticking your head under the sill and shaking the car...
good luck.

regarding the exhaust:
- heat shields are loose, but they make a different sound. By the way, can the heat shields be removed? We have welded them couple of times in last two years, but they always come loose after some time.
- there are no cats - just a straight pipe
- exhaust system from downpipe to back box was renewed last year
- back box is an original one, 15 years old, and it is in pretty bad shape, but once again, the rattle that worries me is like one cylinder is waaaay louder than others.

Just to explain how loud it this rattle: when you drive with windows shut, the rattle CAN'T be heard. Only with window down. So, it's not very loud sound.
Also, I had my GF to drive the car by me few times, while I was standing at the side of the road -> i couldn't hear the rattle from outside, when the car was passing. 

One more thing: rattling is present only on lower revs (1500-2500 RPM). Above that, the back box becomes so loud, I can't hear if the rattle is still present.