... it's just a trill. Nothing technically special about it at all.
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You sure? I thought my trills were reasonably good but it never sounds like some sort of pinball, arcade game.
It's just a trill with a bit of a delay on it.

Use a digital delay at about 200ms and a fair bit of reverb on a clean channel, or very low gain. That should get you a pretty good hendrix tone. You can beef it up a bit using a compressor if necessary too.
Anyone else have an idea of getting that sound?

Sorry to bump an old topic.
Sounds to me like another string is ringing out slightly out of tune with the trill. The trill is just a trill, the other noise is him playing another note with it by accident, possibly a harmonic.