It happens a lot but not every time I hit 55.
What causes my steering wheel to shake when I go over 55 mph? on 2007 Toyota Corolla
Your tires may need to be balanced.
IF, when the vibration occurs , tapping the brakes changes it (or stops it) have the struts checked. AND yes have tire balance also checked because that would be much more noticable , if the strut(s) bad. Tire balance issue should be consistant anytime you hit the specific speed (frequency). A strut will sometimes not be noticable until you go over a bump or something else to 'aggravate' it.