scalescliffs
cashed
I wouldn't say the whistle means you should back off as i have gotten some great hits hitting mine at the whistle . It will eventually tone down and sometimes go away as you build up some resin in the bamboo , mine has. Also filling the hydra-tubes should be done slowly to avoid the over flow chamber. Like you stated , just enough to cover the perc. I would not keep water in the overflow , i think a droplet or 2 is fine but there have been times people have lifted there hydra tubes fast only to get blowback into the bamboo and some of there electronics wet. I personally had this happen when lifting one of my ht's off its stand.
Only reason I mention that the whistling is an indicator to back off a tad is because it's listed as such on the VXN box. I think they are worried that liquified concentrate could be sucked up out of the slit and fall down into the bamboo and get gross in there.