Creaking when sitting and pedaling

Hello everyone.

Bike is a BMC alloy hardtail. BB and cranks are DUB and NX respectively. BB has two services done (LBS says it's not necessary to replace it yet) and 1700ish miles, same with the chain, chainring and cassette. Everything else works like new and has been serviced regularly. Didn't creak from new up to about two months after its first service when it started creaking both sitting and standing. Stopped creaking when standing after second BB service a couple of weeks ago, its absolutely silent, but creaks like crazy when sitting and pedaling, specially climbing on lower gears. Not coasting, not breaking, just sitting and pedaling. I also tried sitting more forward and rearward on the seat, no difference.

LBS insists it can't be the BB cause they just serviced it but I swear it sounds like it's down there, but to be fair it doesn't creak when standing so maybe they're right.

So far tried greasing seat tube and frame contact point, seat clamp and wd40 on the seat rails. No difference. Next is grease on seat rails and full cleanup of the seat tube insides and maybe grease again, and check more thoroughly for cracks, but so far haven't found anything.

I read somewhere that DUB bbs are creaky as hell and would like to verify over here in case I lose it chasing this noise over the next few days.