what are signs of the blower going out?

I had the mechanic take a look at it, and said everything looked fined in the outside.
What i do notice is when my truck is off and the bypass is close, When i turn it the blower sounds like it raddles, until the bypass is open, then it goes away.

 

just checked the oil in the blower and it's perfectly clear,(like new). And there was no shavings.