Check to make sure your spark plugs aren't loose (loose enough to turn by hand). They're only torqued down to about 15 ftlbs (should not be 20+ ftlbs unless they're the redesigned 2008+ style sparkplugs) A loose plug will create a subtle "ticking" noise that will be borderline noticeable. Especially when you listen from the fender well on the affected side. I had originally mistaken it for just being the fuel injectors and dismissed it as normal valvetrain noise until it drove me nuts enough where I used a mechanics stethoscope to probe each of the valve cover bolts. Then after pulling the coilpacks, I noticed that the #6 sparkplug well seem a bit more "carbonized" than the rest which was relatively clean. Stuck a sparkplug socket in there and sure enough, it was loose enough to turn by hand. Tightened it down and it was as quiet as a sewing machine! I had actually misdiagnosed it as a collapsed lash adjuster and was getting ready to pull the camshaft to replace all of the rocker/followers and lash adjusters with a Ford Performance kit off LMR.