Strange rumbling noises above Alkmaar Holland

After a heavy storm I heard a strange constant rumbling, I took my camera into the front garden and filmed the grey sky, it sounded like thunder, it must have been thunder only it didn’t stop for more than twenty minutes, I only managed eight minutes of film as my battery was low but the rumbling and lightning just carried on and on, the rumbling stopped but came back constantly for the next hour or so. I can honestly say I have never witnessed anything like this before.