After getting up to speed i hear a rattling sound undernerath the truck when pressing gas pedal.
My car has 112000 miles.
My car has an automatic transmission.
One of the number one causes for rattling noises on an automobile while driving would be a loose heat shield on the exhaust. These heat shields are made of thin sheet metal and are intended to prevent excess heat from reaching components that are near the exhaust system. I would recommend having your exhaust system professionally inspected to check for any loose heat shields. If any loose heat shields are found, I would recommend having them replaced in order to resolve this issue.