In general terms, the Spanish Royal Family is not liked by Catalans and Basques
FC Barcelona and Athletic Club de Bilbao will play Copa del Rey final. It will be the 8th time they meet in this kind of match and they are also the two teams that have won more Copa titles. However, Spanish governors (as well as the Royal Family) don't like these two teams playing the final.
"This story must end. Whistles could be sanctioned."
Spain have two regions that have historically wanted to become a new state: Catalonia (FC Barcelona) and the Basque Country (Athletic Club). Catalonia did a non-binding referendum last November 2014 where YES to 'becoming a new independent state' clearly won and where some Barça players like Xavi, Montoya or Sergi Roberto participated. Now they have elections scheduled for next September 27 that will be used to determine whether the population want independence or not, as the November referendum was disallowed and annulled by the Spanish government. In the Basque Country thinks are different. They haven't still made important steps to decide if they want to leave Spain or not. But what's for sure is that, in general terms, they don't like Spanish Royal family neither.
So every time the Spanish anthem sounds in Copa del Rey final and both teams are in there, the supporters from both sides whistle so loud that the anthem can be hardly heard. Here you have the last two examples:
La Liga's president, Javier Tebas, warned Barça and Athletic fans: "I'll work to try to stop whistles. This story must end. Whistles could be sanctioned."