how many miles are on the engine? and timing chain if it has been changed? I have had the chain tensioners get stuck and the chain will slap all over the place.
maybe the chain tensioner needs higher oil pressure to apply pressure to the chain, explaining why it only does it at low rpm and even more so under a load.

I have also seen broken guides that dont make noise all of the time, the chain will wear a groove in the cover and be pretty quiet sometimes. The top of the guide may be intack but the bottom gone. I personally think its in the chain.