When I sit on the toilet bowl in the morning, I keep hearing a sound. It sounds like "Gu loook" It sounds lke water or its bubble is trying to get somewhere. It disappears once I flush the toilet (and still sitting on it), Also, regularly now, at end of water tank refill, there is whistling noise. It can last up to about a minute. Once, I even heard drops of water droping inside the tank. What seems to be the problem? Will it turn into a major disaster if nothing is done.

You could just have an air lock in your system... the pipes need bleeding that will clear it.

I see you have two posts so ignore my other post. I doubt that you have an air lock in your supply unless some major work has been done and an air lock would clear itself pretty quickly. It sounds like the ballcock valve is slow to close and may not be closing completely. How high up does the water come to the top of the overflow tube?

If it's the ballcock it can be rebuilt but they can be replaced for under $10.