The on battery light blinks yellow along with the green light blinking and it makes noise when doing it.

Check battery and charging voltages. APC says 90% of faults are a dead battery. My experience also

A blinking green light indicates it's on battery.
Possible Causes:

• Your power is out
• a breaker/fuse is tripped/blown
• the outlet is otherwise not working (GFIC?)
• The cord is unplugged.

So something happened to this apc and the first thing is to replace batteries...
I'm going to guess major circuit board failure.
go to apc site, download Power Chute software and pray there are useful diagnostics.

Without knowing which light is flashing I can only guess. From my experience and assuming it is the battery light that is lit and NOT the overload light, it can be one of two things, one is your battery is near the end of its life and needs to be replaced, and the other is that your battery is improperly connected.

To confirm if your battery needs to be replaced or not, you can run a self test. Ensure that the battery has had time to fully charge (and that it is definitely correctly connected), and then turn off the UPS, and turn it back on. It will automatically run a self test.

if the light stays lit, you need to order new batteries. One source is here: http://www.refurbups.com/APC-Back-UPS-RS-900-BR900-Replacement-Batteries

If it IS the overload light that is lit, you simply have too much equipment attached to the UPS, more than the inverter can handle. Try removing some equipment until it turns off.

A blinking green light is likely an indication that the internal battery is nearing the end of its usable life cycle. If it's over 3 years old this is likely the case. To test, plug something in to the battery outlets and pull the plug of the UPS from the wall. If you get a short runtime (or none at all) it's the battery. Batteries are under $20 and can be found here: http://www.batteryinternational.com/apc-backups-es-be350u-ups-battery-ups-batteries-f1.html

Nope! The internal battery is not plugged-in correctly. Unplug the APC unit and open the access door to the battery (it is not dangerous to do).
Re-seat the two wire clips (red & black) to the battery. The positive post has a red mark on it.
Close it up and charge for a few hours before using.

If the symptoms continue, return unit to store.

Internal fault.

It sounds like you may need a new battery. The batteries are continuously tested by the UPS and it could be that the voltage is passing one test, and failing the next which is resulting in the light flashing. You can test the battery with a voltmeter. It should read over 11.5 volts, if not you need a new battery set. You can get batteries from http://www.batteryinternational.com/ups-batteries-apc-batteries.html

You need to run recalibration on the unit.  The supplied PowerChute software & cable will allow you to do this if you hook the UPS up to a PC (I know it works with the serial cable, not sure about USB).  If you only have a MAC then I cannot offer a solution other than to buy the standalone PowerView device from APC.  It will cost a bit more than a battery, but may save you from buying needless batteries in the future.  I finally broke down an bought one, but note that I was still having the problem after the 1st recalibration.  By trial and error (and maybe a vague memory) I put my MAC in sleep mode, that dropped the load to stop the lights from flashing.  Then I recalibrated with a 22% load and now it finally seems better (lights no longer flashing).  This was with a Smart-UPS 1000.
Let me know if that helps.

not sure but have you checked the backup battery inside