Every time the ac comes on in our house, my mom says it smells like mildew. does this mean we have mold in the vents? or is it likely under the house? someone explain, . you

That is not necessarily correct. If it were that simple, we indoor environmental consultants wouldn't have the need for the other equipment we use to define indoor conditions relative to mold and develop a scope of work for its removal. Try contacting a local indoor environmental professional that has no ties to a remediation firm. They may be able to discuss in greater detail (than one of these forums) how you can determine the need for an inspection, your risks, and prepare you for the discussion you'll want to have with your landlord. There are directories online for these indoor environmental professionals. Use the ones that are independent and accredited like the American Council for Accredited Certification and the American Industrial Hygiene Association. Both of these directories are good ones. Also get some real facts on water damage and mold. The fact is that mold can go dormant, instigated into activity by changes in moisture (which may be why you smell it heavy during a heavy rainfall). Furthermore, the odors do not have to be coming from the carpet in that part of the house because you smell the odors there - it can come from other areas.  

More than likely in the vents

i believe the answer is true

answer; the above statement is (true);