after the breakin of my self built 355 (see sig.) i run it slowly until it was warm and revved it to 5000rpm for just a second.

at that moment a rattling noise came up.

Please tell me its just a rocker came loose. :(

That depends on how you adjusted/set the valves before you started the engine.
I say adjust the valves again, always a good thing to do after you run a new engine
a while, take out any slack (lash) and add a 1/4 turn. (while running is best)

if not the rockers, what could make that pretty loud rattle?

Pull the valve covers and readust the valves. It is unlikely that one just decided to loosen up though. Also make sure that you didn't pull a rocker stud or bend a pushrod. If you find one valve that is really loose, look at the rocker stud and the pushrod. If both of those are fine, you probably wiped a cam lobe. You did run the engine at 1500 to 2500 RPM for the first 30 minutes to break in the cam didn't you? If you find bent pushrods or pulled rocker studs, your valve springs are binding or there isn't enough clearance between the retainer and the guide. This would need to be corrected before you refire the engine.

Did the oil pressure suddenly drop? Was there any other sudden changes? If the valve adjustment looks fine, you are probably looking at a problem in the bottom end.

I side with Cameron, either wiped lobe or pulled stud, or a loose(n) locknut.

i checked the rockers and stuff, all looking good, no wiped lobes either.

man, dont tell me i have to pull the whole engine again !?!

Well, rev the motor from the engine bay and listen to where its coming from. Probably something really stupid its easy to forget little details when putting a motor back together.

Make sure you didnt blow a header gasket also. I pulled all my valve springs, checked all the rockers, push rods, ect. Came up short. Then thought I bent a valve. Did a compression check. Looked great. Assembled everything, started the motor and happen to notice a piece of string from rag had gotten caught under the valve cover gasket. As I went to grab it I noticed the heat from a blown header gasket. Broken a couple valve springs before and can honestly say that gasket sounded just like one. Check the simple things first.