I had a very similar noise and I thought it was a phaser but it turned out to be a bad catalytic converter. It could also be a exhaust manifold leak as my first set of leaking manifolds sounded very similar. Is there any way you could post a sound clip or video.
2000 Excursion xlt 6.8 aka "my big fat ex"
2008 reg cab F350-----sold
US Navy Seabee 07-13. BU2. NMCB 27/22
OEF 09-10 Afghanistan.