My 2003 had a chirping noise and before I replaced the belt (caution, danger ahead) I used a file on the sides of the belt while the motor was running and the noise would go away for a while. Did this for a couple years. Just didn't want to screw with it at the time. Not even sure why it was working
When I figured that one day I was going to eat that file (honey, can you call 911, I got a file stuck in my eye) if I didn't go ahead and replace it and bought both belts. Before putting them on I took a Scotch Bright pad to the smooth pulleys that the back side of the belt rode against to clean any rust or debris off them. Then I used a wire brush on the rest of the pulleys to clean out the grooves.
This is just what was happening to mine and do not know if it could be your current problem with the new belt.
You did change the tensioner and idler so that should take them out of the picture. Might look at the fan pulley if you haven't already. May want to also take a wire brush to the other pulleys.
WD 40 may take sometime to come off the belt unless you take it off and clean it off the best ya can.
Best of luck in fixing it.
Disclaimer, I do not recommend the file trick to anyone for fear of anyone here getting hurt.