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The loud burst of staticky noise could be from either of two causes.
One, you may have the computer on a different power circuit, causing the ground level to be different from that of the 40XL. If so, this could be quite hazardous to the equipment, causing (eventually) damage to either or both devices.
But the more likely cause is AGC at a high level before signal is supplied by plugging in the jack. AGC varies the amplification level to try to emphasize faint input and de-emphasize loud input, and with no input, it tends to be automatically turned up high... thus even a little static electicity on the jack, when plugged in, sounds like a very loud burst.
The cure to that is to have the jack from the computer plugged in all the time, pretty much. The AGC on the input of your 40XL will stay at a moderate level, there will be no burst of static, and you will be able to keep it that way by controlling the sound input from the computer by switching it Off, or On, at the right times.
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