Hey guys first time posting here and was wondering if I could get some help with power chords. I'm pressing them hard enough to where when I pluck each string it sounds good but when i strum all 3 i get alot of vibration sound. I have no idea what i'm doing wrong here. I'm learning the canon rock rythm part and i'm getting alot of vibration and it sounds absolutely terrible Could I be strumming too hard? Not pressing down hard enough? I'm barring the root chord with my pointer then the other 2 with my ring finger. Any help would be appreciated. Thanks!
squeeze until the neck starts cracking.

Nah lol, depends on the tension of the strings mostly and the action. So really its up to you and what feels comfortable and sounds right.
Press just hard enough to save energy. When you are fretting the notes, make sure you fingers are centered (somewhat) between the frets. Press down firmly on each individual note and pluck. If you have buzz, you action is too low for power chord/rhythm playing.
happens with me - only on the 6th string though. and especially when I'm in drop D.
the note is also out of tune if I press the string into the fret too hard.

but in answer to the original question, it may be your guitar - is it fairly new? maybe it needs new strings, or maybe you have the problem I had a couple of months ago - one of the frets is raised too high, and the strings are vibrating against it. maybe take it into your local music shop and ask what they can do.
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To cut down on fret rattle, move your fingers right up close to the fret, don't be holding exactly halfway between frets. Also, holding two strings with your ring finger is a bit of a bitch for beginner. Try using your ring and little finger (positioned one on top of the other) for the fifth and octave notes (the other two, that aren't the root).
Thanks for all you guys responses! Great community here! but my question was why does the power chord rattle/buzz when i strum the chord but not whenever i pluck each string individually. Is this a common occurence with strumming too hard? I'm using ernie ball super slinkys, could this cause it?
That just means that your hand is shifting when you strum, probably because your attention is shifting to your strumming hand and away from your fretting hand.
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Here's a thought. When you pick the strings individually, you may be shifting your grip to get the optimum sound out of the string you're about to pick. Like "I'm playing the first string, I'll hold that the hardest, now I'm playing the second string I'll hold that the hardest..." You may not notice you're doing it. Then again, you might not be doing it :-) When you pick the individual notes, they should ring out and bleed into each other. Don't pick and release and pick the next one.

If it's not that, hold your guitar with the headstock near your cheek, look down the side of the neck towards the bridge (where the strings go into the body). From this angle you'll be able to see if your neck is straight or not. The fretboard/neck should be completely straight with no curvature down the length. Look at the gap between the strings and the frets. This should be uniform all the way down. If you find this is your problem, a guitar shop should be able to rectify for minimal cost if you don't feel confident enough to try it yourself.
yeah, if your hearing "waves" in your powerchords, you may just be out of tune. that is your root note, and its "perfect" fifth, which is perfect and gets no waves cause it is special, and with the three-string chords you repeat the root note et cetera
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I know you will prbably hear this a lot, but it is just a case of practice, after a while of playing, it will feel natrual.