Termites and other pest can cause expensive damage. Here's what to look for: sawdust-like accumulations of frass indicate the presence of termites. An exterminator can spray or set traps to eliminate the infestation. A clean hole without cobwebs or leaves indicates an access hole for rodents. An exterminator can set a trap for them, too.
Clean warm-air registers, baseboard heaters, and radiators as needed. Make sure they are not blocked by furniture, carpeting, or drapes.