2002 ford explorer issues

I have a 2002 ford explorer that I have been having problems with. there is a very loud hissing noise and it will stop when we are using the break and comes back as soon as we let off of them and it is very constant. on top of that we changed the coils and sparkplugs and it is still misfiring. we did find some oil in the sparkplug spots and on the coil. if anyone knows anything about the problems I would love to get some advice. thank you.