Hi everyone. I have a 2010 Toyota Corolla with 75,000 miles on it. Over the last few days, I have noticed that there is a rattling noise occurring in the front passenger side of my car around the glove box area when the heat is on. As soon as the heat is turned off, the rattling goes away. It’s a relatively faint, whistling noise. The rattling does not change loudness or frequency if I drive faster or slower. It only changes with the heater being on/off. I’m thinking it is either something with the car heater or maybe there is some debris stuck somewhere? Any opinions?
My best guess is that some type of debris (leaves, a mouse nest, or…God only knows what) is caught in the HVAC system’s fan, and that you are hearing that debris hitting the inside of the housing as the fan spins.
Just in case it is a mouse nest, you want to get it out and to have the housing sanitized, due to the possibility of contracting the deadly Hantavirus:
http://www.cdc.gov/hantavirus/hps/index.html
It’s easy to remove the blower motor and see if there is anything in it. It is behind the glove box. You need to remove the kick panel below the dashboard if present and maybe the glove box. There are 3 screws holding the blower motor in place. You also need to remove the wiring harness. Once released, you can check for debris, remove it, and reinstall. If the debris is not in the blower motor, you have limited reach to check the vent shaft by hand.
Thanks for your advice, VDCdriver and jtsanders! I will definitely check out the blower motor this weekend to see if I can find anything stuck in there.
Check your cabin filter first, here is a link on how to do it.
It is very easy and you might find the stuff @VDCdriver mentions. I found a mouse nest in my daughters filter.Easy enough to get access and get a vacuum cleaner in.
Please keep in mind that while a cabin filter is a good thing for people prone to allergies, it isn’t necessary for the operation of the vehicle. If you should find that it’s the source of the sound, you can leave the filter out until you pick up a new one… or forever if you’d like. The car won’t know the difference.
I’ve recently checked my cabin air filter and it was clear, but I’ll definitely check that again. Thanks!
If the noise started since you last checked the cabin air filter, it’s a great place to start. Not that it is a mouse, but if it is, the mouse could chew through the filter and move into the vent system. Good catch @“the same mountainbike”
Leaves can fall into the blower wheel if there is a pile of them on the cabin filter when it is removed. I don’t think that is the case here, you described what seems to be a noisy blower motor. It will probably squeak until you replace it. You can inspect for leaves in the blower wheel by removing the cabin air filter and looking inside with a light and mirror, the blower wheel is just below the filter.
If you operate the vehicle without the cabin air filter there is a risk of debris entering the blower motor and HVAC system, these cars don’t have fine screens over the HVAC air inlet to keep debris out.
As much as I’d like to take credit, it was Steve that thought of the cabin filter. 
Does the sound change with changes in fan blower speed? That points to the motor or debris being rubbed by the fan.
Does the sound change with the AC or defroster being turned on or off - i. e., is the sound there if you have the fan going in vent mode, with the AC and the defroster off? I don’t know what kinds of sounds an expansion valve can make, but it may be behind the glove compartment.
these cars don't have fine screens over the HVAC air inlet to keep debris out.
To prevent debris problems from returning, it’s a good idea as part of routine service to vacuum the air inlet vents at the bottom of the windshield of debris. It’s easy to do with a shop vac.