Bike noise, creak, click, creaks, clicks, clicking, pinging sound - help

Hi all, hoping to tap into the infinite wisdom of this forum!

Summary to avoid tl;dr: random occasional clunking noise when pedalling, never when coasting. Possibly freewheel? It has some play if that is a clue.

Can you help me answer:

• Is this a common or bad problem?
  
• Do I need to replace something or will adjustments sort this?
  
• How can I avoid it next time, or will it just happen?
  
  

More details for the keen: Starting to have this problem more and more, which doesn't seem to affect the running of the bike except that it's a bit off-putting and I'm afraid it's a precursor to all sorts of things going wrong so here goes:

When I pedal, there's a metallic clunk noise almost at random. This is not associated with:

• any particular point in the pedal stroke.
  
• any number of revs of chain or wheel.
  
• any particular level of pedal-pressure or cadence (so happens when cranking hard or just spinning light).
  
• any chain skipping or slipping.
  
• any particular sprocket in use.
  
• any particular weather conditions.
  
  

Sometimes it can happen many times in a row, other times it may happen once in a two hour ride. My freewheel is a stock Shimano 6 speed, mf-tz06 I believe. There is some play in it when twiddled by hand but not a lot, and doesn't clunk when you jiggle it manually (yes yes technical terms ;) ) Bottom bracket is smooth with no play, chain is well looked after, as are the mechs. Big mystery to me - hope you can help!