My air cleaner has started acting strange and I am not sure what is going on. The airflow feels weaker than before and I notice a kind of dull smell every time I turn it on. I cleaned the outer screen but it did not help much. I am worried that the internal filter or fan might be failing. I have never opened this machine before so I feel a bit nervous to try anything on my own. I wonder if someone has faced something like this and found a simple fix. I keep thinking that maybe it just needs a deep cleaning but I do not want to break anything in the process. The noise level also seems slightly higher than what it used to be which makes me think the fan blades might have dust buildup. I hope someone here can guide me because I need clean air for my room and this device used to work really well. Any advice would be helpful 
I had a similar issue with my older air cleaner and it turned out to be dust packed along the inner fan guard. I took the unit apart step by step while watching a basic tutorial and I cleaned the guard using a soft brush. The change in airflow was immediate. If you feel comfortable opening it then it might fix the problem. Make sure you unplug the unit before you begin and take a photo before each step so you remember how to reassemble it.
Sometimes the motor starts to slow down when the lubrication dries out. It happened with my basement air cleaner. I had to apply a tiny amount of safe oil to the shaft. You need to be careful though because too much oil will collect dust. If you have never done this before then consider reading the manual to see if your model even allows motor access.
I want to share a simple test that helped me. Remove the filter and let the machine run for ten seconds. If it still smells dull then it means dust or mold is inside the machine rather than on the filter. If the smell goes away then your filter is definitely clogged. Replace it if possible and see if the airflow improves 
I will try running it without the filter to see what happens
There is a chance that the electrical contacts are loose. I found this out the hard way because my unit would not start all the time. When I opened the top I saw a connector that had moved slightly due to vibration. I pushed it back in and everything worked like new. So check for loose wires if it is safe for your model.
If you hear extra noise then the fan might be scraping dust or hair. I once had a small thread stuck around the center of the blade. It caused vibration and made the airflow really weak. When I removed it the unit sounded totally normal again. Shine a flashlight toward the fan and see if anything is wrapped around it.
One thing people forget is the prefilter. It blocks larger dust and extends the life of the main filter. If it is saturated then airflow drops by a big amount. My prefilter looked clean until I washed it and saw dark water coming out. After drying it the unit worked better than before. It is an easy step and can really help.
You might want to check for hidden screws because many units look sealed but are not. When I tried opening mine I realized that two screws were hidden under the rubber feet. After removing them I was able to lift the cover without force. If you try opening yours make sure you look everywhere so you do not crack the casing.
I know this sounds basic but confirm that the room humidity is not too high. My air cleaner began smelling strange when the humidity rose above normal levels. Moist air mixed with dust created a musty odor. After I used a small dehumidifier the smell was gone. It could be a factor worth checking before you open the machine.
A technician once told me that clogged carbon filters can cause dull smells even when the main filter looks fine. Carbon absorbs odors but when it becomes saturated it releases a strange scent. Try removing just the carbon layer if your model has one. If the smell changes then you have found the culprit.
I did not know carbon filters could cause that
I had a situation where the fan capacitor was weak. It caused the fan to spin slow and the airflow dropped. Replacing that small part fixed everything. I only knew this after measuring it with a meter though so it may not be easy if you have no tools. But it is something that might happen with older units.
My suggestion is to clean the airflow channel completely. There was a long narrow area inside my machine that collected fine dust. I used a soft brush and a vacuum together. It took time but the airflow changed dramatically after the cleaning. If your model has a similar space then clearing it out can make a huge difference.
If your device is older than five years then maybe the issue is simply wear. Motors weaken over time. If repairing it costs more than half of a new unit then replacing might be better. I kept fixing my old cleaner but eventually bought a newer one and the improvement in air quality was noticeable right away 