radon mitigation fan running, but u-tube manometer shows no pressure differential

U tubes are quite simple devices. Make sure the tube connection is not plugged. The hole may be the problem, try plugging it then if the pressure Increases you have found the problem if there are no restrictions for air to enter the pressure will be the same on both sides and no difference in the reading.

Our u-tube shows a 2" WC. IMO, the hole is the problem: the pump is sucking air from the basement, making both sides of the tube at the same (basement) pressure. Hence, 0 pressure difference. This will obviously reduce the suction from the subsoil; at the same time, it will be pulling radon-contaminated air from the basement. I have no idea what the net effect would be.

You could get a continuous radon monitor. (Amazon, $130) Not cheap, but I find ours very reassuring.

FYI, a couple of things, from experience with our system. (It doesn't seem like these apply to your problem, but just in case.)

1. We've had the outlet ice up in winter. (Moist air from sump, cold pipe.) When this happened, we showed 0 pressure.
2. We've had problems in summer, where a water bubble developed in the u-tube. This affected the reading, but I don't remember how much. At the time, we were drawing outside air into the basement, and getting a lot of condensation. So, warm air, u-tube in contact with the cold pipe. We now keep the cellar mostly closed in summer, and I spaced the tube out from the pipe.

Systems typically read around 1" WC. The open hole that is whistling is curious. How far from the exhaust pipe is this hole? With that much air being pulled the WC should be higher than 1/4". WC is the difference in column heights not the reading on the positive scale. Add the below zero value to the above zero value.