Let's Play 'What's That Sound?'

For some time now, I hear a pulsing 'woo woo' type sound coming from the exhaust. I have no idea what is causing it. I'm a little embarrassed to say I've driven the truck maybe 10k miles since I first noticed it. I don't think it is getting worse, but it sure isn't going away. I would like to avoid taking it into multiple shops and spending potentially hundreds just to figure out the source of the issue (or maybe not even learn that much). I don't even know what type of shop to take it into for diagnosis...engine? transmission? exhaust? This is the main reason I have just lived with it. I have started to create this thread three or four other times but never went through with posting it, thinking that I will eventually figure it out on my own. But now I don't think that is going to happen.

Some things I have observed about the noise are:

• it becomes audible at 50mph only when in 6th gear. it sounds like it is coming from directly under the bed. I can't hear it echoing off nearby buildings when I drive by with the windows down. I don't think it is as noticeable from outside the truck. At lower speeds and in lower gears the exhaust sounds normal.
• the pulsing seems to be related to either engine speed or vehicle speed. Cruising at 60mph it pulses about 200 times per minute. It's a little slower at 50mph and a little faster at 70mph, but goes away if the transmission downshifts to accelerate and it doesn't matter if I'm turning left or right or driving straight.
• when I let off the gas and coast it pulses faster until the truck slows and shifts out of 6th, it kind of sounds similar to an exhaust brake.

Other things to note are:

• I have no engine codes (I did have a P0456 evap system code a couple months ago, but I cleared it and it has not returned). I have tracked my mpg manually for eight years and notice no changes in efficiency since the noise started
• the transmission shifts fine between all gears, including 6th. I last did a drain and fill on it about 70k miles ago
• Like five years ago I swapped out my muffler for a no-name muffler that produces a bit more sound than the factory one. I have inspected my welds for leaks and can't find any. Can a muffler go bad or fail? I don't know what that would sound like.

Does anyone have any thoughts based on all this? Has anyone dealt with a similar issue? I appreciate any input, thanks.

 

Mods, can you move this thread to another forum that might be more appropriate for my question?
TIA

 

@ColoradoTJ
@T-Rex266

 

Thanks for moving this thread and providing feedback. Can I test for a bad bearing in my driveway?

 

You can, but you need to remove the brakes so you can spin the tire by hand. The pads have too much friction to do it with them in place.

A mechanics stethoscope placed on the axle tube will allow you do hear the dry bearing.

If you do have a bad rear wheel bearing, it's gonna be the passenger side rear. It seems like it's always that side.

 

@ColoradoTJ used the term axle bearing. Sounds like @koditten is referring to a wheel bearing. Are these the same thing?