Changed blower motor--noisy at slow speed

Installed Fanhandler speed control with brand new Baldor motor on existing (older) furnace. At slow speed, the motor makes a high-pitched turning noi se like a helicopter. If I speed up the minimum speed quite a bit, the noi se disappears. Also, if I pull out the air filter, the noise changes and d ies down a bit.
I don't like the look of the existing return connection as it has been shor tened to 14" wide vs. original 16" for an electronic air cleaner (which is now an empty box with a regular air filter in it).
I cleaned the squirrel cage rotor before install and carefully switched the motor so that the support bracket sits as before. The only part I am unsu re of is how tight to set the strap around the motor. Perhaps I over-tight ened this?
I should take some measurements--static pressure for one--but I don't have a manometer lying around.
Any ideas?
Thank you, Dan
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