A few weeks ago, my engine began whistling a bit (siffler in french).  Really lightly and a few seconds only. Not a hissing sound but really a whistling one. I was thinking slipping belt but now that  i get this sound more often and a bit longer, it is not a belt noise.

Yesterday i went to my Suzuki dealer. They tell me my tensioner pulleys were beginning to go bad. Maybe it was the problem. They changed the fixed pulley (cheaper) and the belt (since it was a bit old). Afterward, inside we were not getting any noise. I drove it home. Today i got the same problem.

My mechanic was telling me it may come from the water pump since he was feeling it rough when turning by hand. Could it be the problem?  Could it be the alternator?

I don't want to change every part in front of the engine to find or worst, not find!

What do you think about this?