'Tweet' Sound When Car is Off

My 2006 Monte Carlo has been making a strange ‘tweet’ sound. I only notice it when the car is off. It’s just a single ‘tweet’, a minute or so will pass, then it does it again. What could this be? I’m worried it’s something expensive.

Have you owned this car since it was new, or did you purchase it as a used car?

Does the tweet sound like it’s inside the cabin, or outside?

Does the vehicle have an after-market alarm system?

“Does the vehicle have an after-market alarm system?”

One of the reasons why I asked if this car was purchased as a used car is because all manner of aftermarket modifications could have been done to the car prior to the OP buying it.
That is just one of the…joys…of buying a used car.

My wife had this same complaint about her 2003 Chevy Blazer. I went out to check it and the “little red light” next to her radio was keeping perfect time with the annoying little tweet. I found the source of the tweet and crushed it with a pair of pliers. I later disconnected the wires to the tiny speaker just to make sure the “tweet” was dead.