I started playing with a Harmon mute almost all the time several months ago.
It wasn't by choice. It was the result of a negotiation with my neighbours. I tried a practice mute but it doesn't seem much quieter than the Harmon and it sounds a lot worse.
I only play unmuted in the afternoon which restricts me to Saturday and Sunday unless i have time off work.
Up until recently i found this situation made it a real pleasure to play unmuted and i felt like using the Harmon mute actually helped me develop better tone.
Then the raspberry thing started. It only happens when i play without the mute! It's not like a really hard raspberry. It's more like a really pathetic, airy one.
It doesn't happen regularly. It's not specific notes. It's not just when i blow hard. It can happen at any time with any volume.
It never used to happen so I have to suspect that all the playing with the mute has triggered some change in how i play.
Anyone else ever start randomly blowing raspberries? (Only when playing the trumpet obviously. Anything else is your own lookout.)