first try p\holding a small piece of toilet paper near the blower motor around where you replaced it. put it on high and see if it is sucked into the blower area, if it is you cna get like some small strips of compound to put on the mating surface to stop the leak, don'tuse silicone as it will glue the motor to the heater. then check the seal near the top if you can to see if you disturbed something causing an air leak. pic is enclosed of heater. then if you have to pull themotor out see if htere is somethig small trapped inside behind blower fan the compuound stuff is at auto parts stores. and you put it to the mating surface it is like a caulk typ compound that will stay flexible years later. Other htan that it sounds like an air leak but this is the most logical place and no it may not do it at idle becaue you dont' have air being forced into heater like you do at speed.