Gas and Oil Home Heating Furnaces - Water in ductwork

We recently purchased a townhome built on a concrete slab. Our ductwork on the first floor runs under the slab. The ductwork itself appears to be 4 inch PVC intermittently attached to metal. When it rains, water comes into two of the three ducts. We don't have a clue as to where to begin solving this problem. Outside running along this problem area is a deck built directly on the ground. My suspicion is that the rain is pooling somewhere under the slab. Do we call a construction company to break up the conrete in the house and install new ductwork in the same spots? Obviously the ductwork is not fitted properly, otherwise I would think it would be watertight. Any information would be greatly appreciated.

I think first Id look out side. Too get the water away from the home. Are the gutters clean and working good. Does the water drain away from the unit?
Dont know the job or what you have for the furnace. But 4" pipe for heat duct dont sound right. Also if you did put new pipe in the ground there it would not be water tight.
Get the water away from the home first.;) ED

Thank you for replying. Yes the gutters have been cleaned and the downspout nearest to one of the ducts has had a 6 ft. extension added to it to direct the water away from the house. You were right about the size of the duct, I don't know what I was thinking - the inside diameter of the ductwork is 7 in. and appears to be PVC. We can't even tell where the water is coming from, although the duct that is under the sliding door seems to get it first. Right outside the sliding door is where there is a deck built by the original builder, directly on 2 X 6's laid directly on the ground (no piers). Right now we're thinking about tearing the deck out and replacing it with pavers and hoping that it might help but my thinking is that because the present ductwork isn't water tight, water may still find it's way in. Also, if we're getting water in, we are losing both heat in the winter and air conditioning in the summer. As I said before any thoughts on how to get this fixed are much appreciated.

As far as cost goes kick this around. "What if" under the deck or next the the footing by the the wall outside you put in a sump pump. Dig a small pit and put the pump in.That you could pipe the water away from under the home.We dont ever try and make the duct water tight under a slab.Also if it keeps on it will rust out the steel L's that are down there.
Or hey in the winter you do have a dam good humidifier here but in the summer its a killer when the A/C is on.;) ED