Why is my heater screaming, sounding like something is stuck or fan is quitting?

What is your question?

Problem started a year or more ago and was fairly intermittent, but is worsening. Lately I notice the heat does not blow out as forcefully when the heater is screaming. Could it be the intake air vent for the heater is clogged and if so, where is this located and how do I clean it out? Or is it the heater blower/fan itself?