Just bought a used 2008 Aveo5. When it rains,

Hunting water leaks is a pain in the tailbone job to do. I'm assuming yours has the jack in the floor of the trunk, under the carpet.
The spare wheel well is the lowest point in the trunk so the water will collect there regardless of where the leak is. Start with the simplest tools, a flash light and the palm of your hand.
How much water is collecting and how fast does it fill up? Rain water coming up through the floor is unlikely unless the floor is damaged.
First take everything out of the trunk that wasn't in there when it came out of the factory. Take a really good look at the carpet and trunk side liners. Use the palm of your hand and feel the floor carpet for wetness. Your feeling to see if it is wet in a particular area. Using the flashlight look for darker patches as carpets often darken in color when wet. Move to the rear of the trunk and feel the liner on the back of the rear seats. Next run your hand over the side panels where the wheel arches are. This will give you clues as to where the leak is possibly coming from.
If the whole trunk carpet and liners are bone dry the leak is from under the car.
All the doors, the trunk lid and the sun roof are fitted with rubber seals to stop rain and road splashes getting inside the vehicle. Over time these perish and start to leak. Run your hand around the underside of the rubber seal the trunk lid sits on when closed. This is the most obvious place it will leak from. You can do this on each door as well because water can travel a large distance before it finds somewhere to create a puddle. A Sunroof can also leak but they tend to cause puddles in the foot well.
If you get a few dry days you can test for leaks with a hose pipe. Keep the trunk closed and make sure the inside of it is totally dry. Remove any spray nozzle from the hose so it flows at regular pressure. Carefully aim the water so it only flows over the trunk lid. Let it flow for a minute or two trying not to soak the doors at the same time. Turn off the water and leave the car alone for about 10 minutes. Wipe off any excess water on the trunk lid, open it and see if its now wet inside the trunk. Yes? the trunk seal is leaking. You can use this method for the doors and sun roof as well. Door and trunk seals are usually just a 'Push on/Pull off' fit so you can easily change them yourself.

posiibly you need CV joint.it happens louder when you turn the wheel right?

If your EX has a spoiler on the trunk it may be leaking around one of the bolt holes. That's where they usually leak. Check your trunk seal for any tears.

Depends on if it is leaking from being thrown up in the wheel well, or leaking down from the trunk lid seal?
If from the wheel well, usually a spray can of undercoating will seal it up.
If it is the trunk lid, then re-adjusting the hinges and latch usually do it.
But it could be the seal is misplaced, damaged, or there are leaves clogging it.

If battery leak it is not water but battery acid. If water then there must be leak into trunk.

there may be a few things causing this, first make sure there no open holes in floor, then check trunk seal and make sure its not coming from trunk, next would be the back window,take a water hose and spray diffrent areas and check for leaks inside,could be also rubber around the back door or rust holes in wheel wells of rear wheels,

It is usually easy to find leaks with a hose, because that is higher pressure then rain. But wheel wells are also common places where water gets in, because of the splash pressure.
Don't forget that the fresh air vent drain outside, in front of the windshield can also get plugged. This will bringin water through the heater.

Similarly with the trunk, have someone get inside with a flashlight, and use the hose to simulate rain.

Most of the problems come when its raining and if its not being driven and water still comes in then its the bonnet rubber and if when its being driven then its due to the rear wheel throughing up water under wheelarch and somewhere theres a crack in the weld or somewhere and it will find its way in..
So if not using it clean it out and when rain stops look inside no rain no leak Have a goodday

could be the drains but most likely from spoiler leaking past the seals or wheel wells rusted out or caulk from seams is it when you wash car or rain it rains check it out use a garden hose for water test

same problem happened to my 2005 bonneville. inside the trunk and behind the carpet covering the wheel wells there are two water drain tubes. these tubes run down the inside of the "c" pillar and drain the water from the sunroof. they make a 90 degree bend at the bottom back of the wheel well(inside the trunk) and fit inside a round rubber tube/grommet at both of the rear wheel wells that lets the water drain outside.either the plastic 90 degree tube has come out of the rubber tube/grommet or the drain tube could be plugged up with debris. also make sure the two round rubber tube/grommets are installed correctly and seal the hole going through the trunk to the outside of each wheel well. this is of course only for those bonnevilles that have a factory sun/moon roof.