2011 Pre-Runner Growls at 1200 RPM

I have less than a 100K miles on my 2011 2WD, Auto, 4-door, V6 Tacoma Pre-Runner. It's the best vehicle I've ever owned and I take good care of it. Don't tow anything.
It's a basic model, no frills but I've added several mods thanks to Tacoma World.

Always been trouble -free, however, the other day I noticed a distinct growl at 1200 RPM. I thought maybe it was an exhaust leak but I haven't seen any holes in the system. It's happening every time I drive now.
When I accelerate, it will kind of growl at 1200 RPM then kinda go away above or below that RPM. The noise returns if I decelerate or cruise at 1200. I changed the oil myself and use OEM products.
I've never messed with the transmission and I'm not sure if I want anyone else to.

Has anyone experienced this or have a suggestion?

Thanks.

 

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Check serpentine belt and idler pulleys. also check components on the serp belt line for issues.

 

I'll definitely check those. What should I look for?