i have a pair of YORK furnaces installed in my basement. Once a while one of them starts making unbearable rattling noise - maybe once every couple weeks, and the noise lasted, i don't know, hours?
I looked for the source of noise and found it's from this little guy - see below
Little Giant Pump Company, Oklahoma City, OK
My apology that I couldn't provide more info as I don't know too much about furnace. Could it be the weather being hot so that the furnace is overloaded? However any advice is appreciated.