Hatsan Flash QE .25 caliber much louder than in .177?

Not much experienced but as discussed in another thread, yeah but depending not so much on your moderator -- good moderators can make you hear nothing but the ping of your gun -- but the background. A harder hit, maybe with a bigger pellet I dunno, one or both of those, is going to make a bigger sound than your rifle's noise. 

My .25 is certainly much louder than my .22 than my .177. And they go in descending order of quality rifles. The cheapest mis my .177 Avenger.

The moderated Cricket .25 with a DonnyFL Emperor is MUCH louder than my unmoderated .177 Avenger. Simply because the former hits the target, punches through the plywood behind it, and hits the tree behind it so much louder than the unmoderated Avenger .177 can even attempt to do.

And the KOI moderated Prod is no different. Notably quieter than the Emperor'ed .25 Cricket and noticeably louder than the .177 Avenger even though the .22 Marauder pistol/carbine has the DonnyFL Koi on it.

If they were all shooting into pillows or something, maybe a different story. But you can't discount that after a certain point, no matter how quiet the rifle is, the impact makes noise too.
 
You use a bigger blast of air to shoot heavier larger caliber pellets thus more noise due to shot blast and 177 doesn't require much are when compared to 22 and worst 25 that's the loudest biggest blast of air. Takes a lot of air blast to shoot a much heavier 25 caliber pellet. That's why you get way less shots and much louder sound the larger caliber you go.