[MaciekA]

Often this noise actually comes from your main spring. Acoustics in airsoft are a bitch .. .. A lot of the time if you can't solve a given acoustics issue with memory foam, grease or bearings (i.e. bearing spring guides, bearing piston heads, etc), it comes down to the actual receiver or gearbox shell, and those are expensive to replace.

If it is indeed your charging handle spring resonating with the vibrations from your shots, I'm not sure there are many options side from padding the space above or below it with small bits of foam insulation or perhaps replacing the spring altogether with one that doesn't resonate or sing.