GM Truck Blower Motor

'94 C1500 PU. Just recently,when I turn the ignition on, what sounds like the blower motor fan hits the plastic housing causing a clack...clack noise.
I put my hand around the housing and it was very eveident that the noise was coming from there. This goes on for about 1 minute and then stops. The air flow is fine. When I turn the ignition off it does the same thing with the noise. I'm thinking that as the fan speed builds up and decreases it's balance is off, hitting the housing. My manual says to get at the blower motor I need to remove the panel above it. I think I removed every visible screw holding that panel on but it's staying put. First, do you think it's the blower motor that needs replacing? Second, how does one get that panel off to get at the blower motor?
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Probably just debris in the squirrel cage fan.
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