I found this explanation, which I hope may help you understand why you have feedback, and how to possibly fix it:
It's easy to forget what a wonderfully expressive instrument the electric guitar is. Oh well, shredding won't last forever.
Intentional feedback is an important element of the electric guitar sound (which by the way, you can never get from a plug-in).
To get feedback, you need to turn the amp up, not necessarily all the way to eleven, but far enough so that when you hold the guitar near the speakers, the sound from the speakers makes the strings vibrate, which reinforces the signal from the guitar. Feedback is controlled through the position of the guitar, damping the strings with the edge of the right hand, and subtle left-hand finger pressure.
But some guitars feed back uncontrollably causing a fierce whistle rather than anything you could possibly call an expressive effect. What causes this?
The answer is that the coil of the pickup is, or has become, microphonic.
Coils degrade over time. Even without mistreatment, the lacquer that insulates adjacent turns of the coil will crack causing microscopic short circuits. But the worst case is when the winding isn't uniformly tight and some turns of the coil have become loose and free to vibrate. This is what causes uncontrollable feedback.
The solution to this is to 'pot' the pickups. This means dipping the pickup in hot wax so that all of the turns are physically sealed. You can easily use the power of the Internet to find out how to do this. But try it on a pickup you don't mind damaging first, not on your vintage Stratocaster.
With potted pickups, you should be able to hold your guitar as close as you like to the speakers, while you remain in control of the degree of feedback all of the time.
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