2004 Suburban 5.3L when the engine is cold or has been sitting for awhile and the vehicle is started there is a rattling sound not metal rattling but once the vehicle and fluidswarm up the noise stops

I have a 2004 Suburban 5.3L when the
engine is cold or has been sitting for awhile
and the vehicle is started there is a rattling
sound that sounds like it is coming from
under the passenger side value cover. It
does not sound like metal on metal once the
vehicle runs for a few minutes giving fluids
time to warm up the sound stops.  Checking
the guages on the inside. The oil pressure
guage jumps up to between 40 and 80. And
stays steady around 60 under idle. Now
when normal driving the guage quickly jump
up to around 80 and as you start to coast
the guage goes back to around 60. The
guage actually fluctuates more the
tachometer. Under regular idle it stays
around 60 or even a little higher. With a slight
touch of gas right back to pegged out at 80
or slightly over 80
My thought is the oil pressure sending unit
any other thoughts or help figuring out this
issue would be a great help. Thanks in
advance!!!