if you have checked your bungs are not the issue , first check along the underside for any cracks , also any water pick up pipes for fish box or water pick up lines as these may be a flush flange thats come loose ,  if nothing is obvious i would flood the bottom of the hull ,  carefully in stages and mindfull of the weight on your trailer is carrying as well as hull   IMO 

 This is how I do it.    Sounds like its on the keel line. Put 50 odd litres of water in the hull and add a fair bit of food colouring any  colour till the water is  very dark.  Up and down it on the jockey wheel to move the water around. If you see coloured water leaking on the bottom its pretty easy to find the spot.  One place where they always end up slow leaking is the pop rivets holding the bungs in or the bung itself. Just hope its not a crack on the keel line.  

 Agree with cruzy111

Had a leak in my bayhunter that i couldn't find, put a couple of inches of water in the hull and added red food colouring. Left it for the weekend and found a pink stain on the keel line, once found any easy fix for a Ali welder

Good luck

camzak

 food colouring in the hull with water. Wait for it to seep.

 thanks guys definitely not the bungs. I had a leak via the pop rivets on the bungs last year where water was coming in via the centre of the rivets so they were fixed and I checked yesterday and they are fine . Thought of the water pickups but they seem ok and water not seeming to be coming from that area. 

Any recommendation on good welder nth of river?? 

 flooding is one way but an easier way is go fishing lol go out for a while dont use bildge upon return leave bung in go home try park on dry pavement and let sit for a while every now and then raise and lower jockey wheel and look for any damp spots on ground. after a while drain any water out and let boat dry look over the hull if there is a leak you will notice a salt build up where it is

 Does a top job

 well like they always advise.... Check all the pumps and fittings in the transom when looking for water leaks!!!  I went ahead on my path and  filled the boat with water and found nil except slightly dripping bungs.. So I replaced those. No cracks.. Then dropped the boat in at hillarys with all the back floor up and a piece of the floor in the midldle up. It was hard to see but found.a leak on my transom fitted live well pump near where the outlet hose clamps on. Thought originally it was the clamp but it ended being a crack in the body of the pump where you can't see it. What made it trick to diagnose was when the water leaked it would run down a strate towards the bow and then drop down to the keel and flow back to the transom. So it looked like a leak up front somewhere. When the pump was activated it would piss in water. (The pump is also used for wash down). When it wasn't on it would trickle in water.. It was only about a half inch hairline crack.  All fixed/replaced now but lesson learnt .. to quick to think it was a crack in the integrity of my hull spent lot of hours checking this when should have had closer look at transom pumps and fittings.

Best way is to place water with detergent in the hull (bearing in mind the above trailor wt etc precaution). Then pressurize the hull, either through the bung or in my case another in cabin "bung" access to the hull space. Best way is a Tyre pump or any other pump(just find some way to seal). The hole will become obvious as bubbles will form from the smallest of holes. In your case it would have bubbled at the bung and easy to spot. Adding some colour to the water/detergent mix might just make it easier to spot coloured bubbles.
A tyre on the trailor blew between Kcity and Exxy, and caused the mudguard to flick onto the hull leaving a ding in the ali hull so wanted it checked prior to trips off Exmouth. Nice old timer marine mechanic up there showed me the trick, and the hull was sound (put did still patch the ding eventually).
I check this every time there is more water coming from the bung at the end of any trip. Usually it is always the bung not tight enough.

 My boat is an old stabicraft, which has 5 bungs, each going into different sections, 4 sealed sections, and the main hull. I was getting similar amount of water in, and I chucked the hose straight up the Bung hole till it was fully full in the section, then the pressure made little water fountains on the pin holes in the deck. The down side is it also filled up somewhere up the front, and the water is not draining out, and the deck is welded aluminium checkerplate, so I will have to put in some inspection ports into the deck to get the water out. Never a pleasant thing, cutting holes in the boat. 

Use a garden blower in the inspection port, wrap a towel around the nozzle.

Air will escape, but the pressure inside the hull will be enough to detect a leak on the outside using a spay bottle with soapy water.