Looks like my iPhone vibrates a little bit faster than supposed

Hello guys, I recently bought an iPhone 4 used from US and I noticed that it vibrates in a strange way, it really looks like it vibrates with more "strength" than the normal, I have compared to another iPhone and even the sound looks different, it looks like it's faster or something. I tried to use another vibrator from other iPhone but still did the same.

crwdns2934089:0crwdne2934089:0 crwdns2934093:0crwdne2934093:0

crwdns2934109:0crwdne2934109:0

crwdns2889612:0crwdne2889612:0 0

crwdns2944067:03crwdne2944067:0:

Please? Anyone?

crwdns2934271:0crwdnd2934271:0crwdne2934271:0

bump........

crwdns2934271:0crwdnd2934271:0crwdne2934271:0

Did you compare it with another iPhone 4, and were they both purchased around the same time? Phones will often vibrate with more strength when new.

crwdns2934271:0crwdnd2934271:0crwdne2934271:0

crwdns2934285:0crwdne2934285:0