Chain "vibrates" on higher gears My freewheel that I have installed myself only 1.5 years ago has totally worn out by the moment and so I just had it replaced along with a chain at the workshop. I'm still concerned though about the previous one's extremely short lifespan and thus suspicious about my current setup as well.

It's 7 gears freewheel on a folding bike (no front derailleur), and the problem is that it's feels very smooth on 4th gear. When you switch on higher gear up to the 7th, you start to feel some strange vibration as you pedal, which, I suppose, may indicate that the chain is not properly aligned and will wear the cogs out quickly.

Am I just being paranoid? Apart from that, everything seems fine now - gear switching is smooth and under load they don't switch lower with a distinctive noise, as before with my old freewheel. Should I try to play with cable's tension to get rid of that vibration on the higher gears as I use them more often than lower ones?

Thanks.
"Vibration" could indicate a bad chain angle or a misadjusted derailer.  On the other hand, it's typical to have smoothest operation near the middle of the cluster and some very slight noise/vibration higher and lower.  Hard to say whether you have a problem or not without knowing the precise nature and severity of what you're experiencing.

Note that a chain will last roughly 2000mi or 3000km (+/- 50%).  After that it's sufficiently worn that it's beginning to cause excessive wear on the sprockets.  A rear cluster will last roughly 5000mi/8000km under normal use but will wear faster if a worn chain is being used.  Also, if you're on your third cluster your front chainring is likely fairly worn, though the condition of a single front ring is usually not a major issue.