Fluttering Sound When Accelerating at any Speed

My 2009 Prius (205,000 miles) recently started making a fluttering sound. I can't get the .mov video to load but it sounds similiar to a plastic bag fluttering in the wind only it's coming from the engine compartment. It will do this at any speed as long as I'm accelerating heavily or resuming to a set cruise control speed. At a constant RPM all is good. It started this soon after I changed the spark plugs. The plugs corrected a loss of power issue. It was dropping to a lower gear while cruising at freeway speed. I don't think the issues are related but it's possible. I checked the spark plug coils and they are fine. Ran through a tank of treated fuel injector cleaner and will try Sea Foam next. Any other suggestions? I see similiar threads with no final solution. It's not the wheel bearings. I know what that sounds and feels like, I've replaced each of them once already.

 

On the way home tonight it started squealing just before the plastic bag in the wind sound. Once the plastic bag sound started the squealing would either stop or I just couldn’t hear it.

 

No I have not. Can you point me to a description on how that is done? The one belt that I can see does appear to be worn and cracked.

 

I guess I figured it out. Looks simple enough. Would the water pump or drive pulley potentially make that noise or is it more likely the belt? The tension seems fine to me but it’s hard to gauge. I’m going to try replacing the belt first as it appears quite worn and cracked anyway.

 

Anyone have a link to directions for this? I’m leaning towards the dealership taking over based on the YouTube video I saw but maybe I’ll try it.

 

That’s the dude. Looks expensive. I’ll call on Monday and ask for a quote.