1999 Ford Expedition making noise at 45-50 mph?

I just bought the truck and noticed that when I approach 40-50mph it makes a "brrrr" sound and slightly vibrates. The vibration is subtle. This doesn't affect the trucks performance and there's no loss of power. I'm just curious if anyone else has experienced this with their expedition. The truck has 85,000 original miles and appears to be in good condition overall.

Which is better straight pipe or flowmaster outlaw?

I'm driving a 2006 5.4 l Ford Expedition Eddie Bauer edition and I was wondering if I remove the muffler and put a straight pipe would it damage the engine or would a Flowmaster outlaw be better on the engine

Bad ignition coils damage transmission?

I recently had some really bad jerking issues with my Ford expedition. The code came back as a coil issue, so I had the coil packs changed. However, as soon as the packs were installed, I put the truck in drive and it literally would not accelerate. The RPMs would go up, but the car would remain still. Basically there's a lot of revving, but no increase in speed. I then had the codes read and it stated there was a transmission issue, a 'shaft circuit speed sensor' code. I had this part installed and the truck is still not shifting gears although it is in drive 1. It will drive in reverse, but not forward. How did I go from a ignition coil issue to a full blown transmission problem?