I have a stereo brand jvc mx-kc68 When turning on, the radio makes a hell of a noise after 15 seconds What could this be?

In my experience the JVC brand hasn't maintained it's early promise of quality and I avoid the brand these days.

It would have been helpful if you described the noise and mentioned whether it was constant on both FM and AM. Excessive noise isn't unusual on AM due to the inability of the basic circuitry to reject the various sources of interference found in the average home, partly because the AM ferrite rod antenna is usually inside the radio. The average AM radio is highly receptive to receiving and amplifying lots of interference from many sources especially that radiated by fluorescent lighting, cfl bulbs and even nearby mains wiring resulting in a loud hum or buzz.
With the high popularity of FM and the introduction of DAB that has increasing popularity, the development of AM receivers ceased several decades ago.

FM is immune from that type of interference but is by no means able to reject all interference and a powerful signal from an external antenna produces the best result, the best signal to noise ratio.

Assuming the radio isn't faulty, the problem is most likely interference. If the power cord has a ground conductor it is very important it is connected to a good electrically quiet ground. If this isn't possible it could improve things to use an independent ground to the outer conductor of the coax cable to the FM antenna.