A couple of comments about all that is said...m
A few people have used the term trickle charger. DO NOT use one. Instead, use a battery tender. They plug into your coffin nail lighter and monitor the battery and only charge it when it drains to a certain level. This is a healthier way to maintain the battery. I have a BMW tender that I paid about 80 bucks for at the dealer and one that I got from Sears for 20 dollars. They both work great.
Regarding the noise: some folks who store their cars, start them once a week and let them idle for 5 to 10 minutes, which helps prevent all of the things that Luca mentioned. When I first had this happen to my Z, the dealership told me that it was better to drive the car than to idle it. They recommended that I take it easy during the drive and not to jump on it. So I did just that and the noise went away. Now, I try to drive my stored cars at least once a week. The side benefit to the drive is that it takes my mind off the fact that I have what amounts to a wad of cash sitting in my garage and I bought it for a reason-to drive and drive fast.