Not even close.
The noble class was NOT declining in wealth and power at the end of the 11th C. The rise in cities does not indicate a concomitant loss of power in the noble class as you put it.
There were a lot of reasons for the crusades. One minor reason was that Jerusalem was held by Muslims. Of course Muslims would have no problem if a foreign imperialist power like say, Britain, had conquered Mecca and didn't allow Muslims to do their annual pilgrimage. Why would you not think that Christians would be offended if a foreign power controlled Jerusalem and that they weren't allowed to go there.
Now it gets more involved than that. You can see local Muslim powers allowing a few bands of Christian penitents to have access to Jerusalem but as Europe gets wealthier and more people go then it gets more problematic. It gets even more problematic when nobles go there (either out of personal piety or as punishment for a deed in which saying 3 hail marys just didn't cut it) and they bring their entourage. It get's even more problematic when a large part of this entourage are not penitents themselves. You can easily imagine scenes where a lord marches through a town with scores of bodyguards and servants and there is a dispute with muslim locals. Have enough problems and you can see why muslims started putting limits and then bans on travelling to the holy land.
Nonetheless the claptrap that the crusades was simply an attempt by the European noble class to