lately when I start the bike cold and I am putting on my gear I can hear this "hollow" sort of sound coming from the engine, like its down a drainpipe. it doesn't happen constantly but is noticeable.

I fitted dna stage 3 airbox cover a while back but I'm fairly sure it's not that as it has only been noticable recently and only happens when the bike is cold.

Hoping someone has experienced this before?