A vibrating type sound under the truck bed at the speed of 2o to 30 sound stops

Possible exhaust system vibration 'frequency' at lower speeds that changes at higher speeds/engine rpm... If you can reproduce this noise by holding brakes with truck in gear and throttle engine a bit... With a helper, may find the source!.. A drive-on vehicle lift is going to be the best option!!... May be a simple pipe hanger!!!

I may be wrong on this, but just want to get you to look at a possibility. loose heat shield. The catalytic converter uses a heat shield to prevent heat transfer to the body above & combustibles like dry grass etc... below it. Over the years it rusts & becomes weak, then begins to distorted in shape from extreme heat & cooling. The shield is only tack welded in place & those tacks begin to break. While it is cold the shield my be in one shape holding tight, but as it heats up it expands & is distorted, causing a rattling or buzzing sound. This has absolutely no effect on the catalytic converters performance what so ever , and the converter itself does not need the shield. It is only put on for safety purposes. Hoping this is the cause & not something expensive , a couple of large/ long hose clamps " END TO END" & wrapped around the converter & tightened will solve it. LOOK https://www.youtube.com/results?search_query=catalytic+converter+heat+shield+noise+on+Silverado+