If your smoke detector or carbon monoxide detector is sounding an alarm, you need to take action immediately to check for smoke or in the case of a carbon monoxide alarm clear the building and call the fire department to check for dangerous CO levels.   However if your smoke or CO detector is only beeping occasionally then it is likely a trouble condition that needs attention.  It may simply need a new battery or to be cleaned.  Or it may be time to replace it.  Depending on the model, smoke and CO detectors should normally be replaced every 5-10 years.

• Low battery or dead battery?  Many detectors have a back-up battery so they continue to operate during a power failure.  Check to make sure the back-up battery is full and fresh.  Sometimes batteries that sit around may lose their charge.  No-name or off brand batteries may not work well.  Replace the battery with a new, quality brand name.  A simple way to avoid late night beeps from dead batteries is to replace them once a year at a time you can remember.
• Check the battery drawer - make sure the battery is inserted correctly and the battery drawer is closed all the way.
• Loose connection - This is rare but a loose wire could be causing intermittent power to the detector, causing a beep that's more random or happens occasionally.  You'll need to  turn off power at the circuit breaker, then remove it from the mount and check the connections.  This should be performed by an electrician or a home owner knowledgeable in electrical wiring.
• Old detector or malfunctioning detector - Smoke detectors normally last about 10 years and carbon monoxide detectors normally last 5-7 years.  If your detector is old it's time to replace it.  This is a life saving device, don't be cheap.
• It's dirty - Gently vacuum to remove build up of dust and insencts to prevent false alarms or failures
• Power interruption - a smoke alarm may alarm briefly when power is restored
• Shared Circuit - Smoke detectors should not be in a circuit with other appliances.  A larger load turning on may cause a power blip triggering a brief alarm.
• Furnace Turned On - If the furnace has been off for a long time, dust or residue may be blown around when it is first turned on.  Open windows to bring in fresh air and see if it goes away.  If it returns get it checked out.