Wheel bearing can do this but it's noise made is localized and not easy to tell unless you have a runout gauge.
Brakes can also squeal - but do so when applied - when they are at rest, they just ride along with little tension or surface pressure - they'd make a scraping sound like dragging or fingernails on chalkboard when you apply the brakes.
CV joints can also make a noise but start out as a low rumble - then in coast - no load or throttle - they groan and even vibrate your steering wheel if really bad. Leaky boots usually do this. Take off the front wheels and inspect - even inside the rims of the wheels they might get coated with the CV joint grease.
Did they check the exhaust manifold?
There is a heat shield, and the manifold itself uses several hangers thru the system - Heat shields can rattle loose over time and if a hanger "bushing" is worn out, the metal bracket that holds the exhaust pipe standoff, can rattle against the bracket.
The "
causes my fiesta to rev extremely harder than normal, " part makes me think it's might be the water pump or timing belt tensioner inside the motor - which can set off a knock sensor making the engine sag under acceleration because the squeal causes the sensors to send a noise to the PCM, which cant figure out what to do so it "defaults'" to a low advance level of timing thinking something is not right - which makes the car seem sluggish.
The torque of the engine on it's mounts - that motion or shift - can twist the exhaust system and pipe to press against those mounts - making a noise travel thru the car that well, - making not unlike the sound of 4 baby bobcats fighting over a stinker - one of them left behind in a litterbox - the smell (or noise) goes away eventually, but not without a ruckus.
Ok, it's over, job done!
Time for a nap...
