Sound when pedalling I have got a new mountain bike(hardtail wtih powerspline bb) and it is on 100 kms. When I push the pedals I hear little sound from the left side. It is only from the left and I think it can be the crank or pedal?! (not sure if it can be the BB as long as I hear it from the left). The sounds comes when the crank is almost in maximum down position. 

The interesting is that when I go out for a ride the sound/creak comes after 8-10 kms. After that I return home>clean the bike>go out for next ride>it comes again after some kilometres. It is very strange. Is it possible to be something different, like mud from the tires or something like that? 

I have checked the cranks and I think they are tighten enough to the BB. I am not sure about the pedals, however. Will it help if I apply on them chain dry lube and tighten them?

Thank you in advance!
Maximum down position is odd - that's when the power stroke is over so its not the pressure causing your noise.  Did you assemble it yourself?  Could be a loose pedal, or a pedal bearing needing lube.

Try popping the chain off the chainwheel and pretend to pedal.  See if the noise still exists.  If yes then its not your chain.

Finally - since your bike is new, have you taken it back to the shop for a tune up?  Almost all bike shops offer a tune after 6-12 weeks to fix up cable stretch issues etc.  A minor creak like this should be covered by that, or by warrranty.