what causes that accidental 'vibrato' sound?

I thought overtones were higher? The sound I get is usually lower than the note I'm trying to play...

I've actually had a lot of problems with subconsciously and accidentally changing the octave I'm playing without the octave key... I don't know how or why I do it (only been playing 4 weeks but been gone for 2 of those lol). I just notice that sometimes I 'forget' to use my octave key but I got the melody in my head and I just copy it anyways and sometimes it takes me a while to realise i've forgotten to use the octave key again.

Either way I'll look up overtones and see what I can find.

Thanks tjaart! I think that's exactly the sound... it sounds like a boat motor sort of lol... and yes it happens a lot more in the lower range (especially coming from a much higher note to a lower note) but occasionally it'll happen above middle c and it's a wretched noise... now at least i know what it's called lol

thanks guys!