Engine Rumble, 1995 325is

My 325is seems to rumble very noticeably when I first start it up. After A while it settles down, but not 100%. This just started happening 3 or 4 days ago. It seems like there is a bad tank of gas in
it. Or maybe it's misfiring. Or could it be the belts or the spark plugs? Last time I had it at the mechanic he told me the belts were going but I didn't want to tack that job on to the already large bill. Does anyone have any idea what could be going on? Thanks...
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Main bearings, big ends?
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New fact. I noticed today that the idle RPM is a tad lower than normal. Is this something that a car like mine goes through once it hit 100k miles?
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Now it seems to be getting better. After about a quarter mile slow drive, the car seems to settle down... Very strange...
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