I have some trouble with my Fender Pro Junior:

1. When I turn on the amp there comes a sound which can be described as some kind of "feedback". It appears shortly after the amp is turned on and lasts for about 10 seconds.

2. Sometimes there comes another sound which sounds like high-pitched scratching. If it appears the only way to get rid of it is to turn the amp off.

Both sounds also happen when there's no guitar or effect pedal plugged in.

I bought the amp 5 months ago second hand. It had 3 stock tubes (Fender Groove Tubes: 2* El84, 2* 12AX7) except for V2 (some Groove Tube with a orange sticker). It worked fine until now, the amp hasn't been really moved except for shipping.

Any ideas what could have gone wrong?
Is there any way to proof if the tubes are working right?
It certainly sounds like an issue with the preamp tubes to me. To check for microphonic preamp tubes, carefully tap each preamp tube with a pencil whilst your amp is turned on, and listen for any unwanted feedback or noise. Obviously, you need to remove the tube shields from the tubes if your amp has these, and make sure you exercise extreme caution working with a live amp - you WILL kill yourself if you are careless with valve amps. As far as testing for faulty tubes, unless you own an old tube tester from back in the day, and have had it fully serviced and calibrated (which can end up costing as much as a small amp unto itself), the best way to check for tube faults is to replace each tube one by one until you find the offending tube(s).
Fender AVRI '62 Jazzmaster, Fender Gold Sparkle Strat (custom), Custom Swirled Roswell Rhoads
Marshall DSL15H, Weber Speakers, Fender Champs (x2),
'63 Fender Reverb, Mosrite Fuzzrite, Joe Bonamassa Fuzz Face, Whiteface RAT
Vintage Tubes!