Only had like 2 hours on it. When got it first thing I did was run it for 20 mins with no tank attached. Then I did the pressure test to see if any leaks. It was holding pressure at 4495 with no problem. Then I tried to fill my 74cf tank. About 40 minutes through it began to vibrate really hard. After that it just build pressure really slow. So I stopped it and did leak test and it was then leaking air. All fittings are tight. It seem to be coming from the head cylinder.
http://m.youtube.com/watch?v=x1HY9NQx1Hs
http://m.youtube.com/watch?v=oXkHuAa0RBs
Any bubbles showing when you applied a leak solution? Seems like mine makes that same hiss noise, and the only place I had any leaks was at the fill whip connection at the bleed block and later on at the opposite end of the fill whip, closest to the tank. During a fill I noticed it was taking too long to build up pressure, so I started applying leak test solution to all fittings and joints, and finally found that one. It was at the crimp to the fill whip hose, just a fault probably when the whip was made. Hatsan replaced it no problem.
ShinyKnight, as soon as you get your answer from Hatsan, please let all know on this tread, to assist us lightning owners