when braking normally everything is fine and wonderful. however, when i break hard there is a low grumbling/grinding sound coming from the engine area (not sure where since I'm driving

). There's no screeching sounds like the brakes are bad.
this first happened yesterday on my way home from work while braking on the highway and has continued today.
any ideas as to what this might be?
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2008 Xterra 4x4
1986 CJ7Lots of custom stuff...
Gone, But not Forgotten...
2003 Sport - 'Fire Engine' RedDetours KJ Backbone
Skids
Boulder Bars
JBA 4.5 A Arms
245/75/16 Wrangler Duratracs's on Cragar Soft 8’s
Spidertrax 1.25" Spacers
Superwinch LP 8500
Frankenlifted