When I just got back my elantra 2001, I noticed that the noise level
from the cabin fan is
significantly louder that the previous level. However, the manager at
the shop said that it's normal and they didn't touch anything about
that. In a nutshell, I trust him, however, what's going on with that
noise?

Here is the symphtom:
When I turn on the cabin fan, there is noise which is from a small-
scale tank caterpillar or cricket or a blade striking a sheet of
paper. When I increase the strength of cabin fan (for example,
1=>2=>3), the noise level increases accordingly.
I guess that the noise is from in the blower unit (containing the
blower motor).

What's your idea? And, what do I have to do to solve this problem at a
reasonable price?
If I bring this issue to a shop, then how much charge is reasonable?
(*Unfortunately, the warranty has been expired.)

Many thanks to you guys.

 

Most likely you have a leaf or other debris in the blower fan, making the
noise.

You have three options:
1. Don't worry about it. If you cannot feel a vibration, whatever's in
there isn't significant and will eventually deteriorate or blow out.
2. Remove the screws holding the blower assembly into the bottom of the
blower box and remove the offending debris.
3. Have a shop do choice 2 for you. Expect to pay 0.5 to 1 hour labor.