If you have a strong head demagnitizer such as Han-D-Mag, try it on the record head and the rest of the tape paths. When the heads get current surge such as when you would remove and reconnect the heads with power on, then the heads will be magnitzed and create the scrapping or crackling sound when playing back. It usually damages the tape as well.
So if the tape that plays fine should be tried on another machine. If you are lucky, it will have the same noise and demag of the heads will the answer.
Hope it helps.
Ki
