just use the mercedes sealant that is used on all the new engines.
As far as the hissing noise, try a couple of things. First remove the belt and make sure the noise is not drive belt related or an accessory. Second did you check the crankcase ventilation hoses on the bottom of the intake? You can also spray brake clean or carb cleaner to check for leaks.
If you do not find anything also are you sure that you did the timing chain tensioner correctly. it should be disassembled and reset and installed without the spring in. Then install the spring and secondary cover. Otherwise your noise could be to tight of a timing chain.