You've checked those twice,... unless you think you could be wrong twice, I think you're wasting your time...
A loose drive pulley noise will increase under described load...
Double check that,... crank it hard with a 32mm socket... you will need a buddy to hold the bike upright and hold the rear brake on while you crank... it has a fold over washer on the nut, but don't trust it to tell you if it's loose...
If the sound in your steth says low in the case, I'd rule out valves and de-comp altogether...
I keep pushing the drive nut theory,... 'cause that booger has fooled three times this year...
... Twice I thought camchain, and once I thought shift dogs... two different noises, but both were from that nut...
...one more thing,... any gas smell in your oil?...