2001 chevy blazer lifter noise problem

I have a 2001 Chevy Blazer that when it sits over nite and is first started in the morning it sounds like a sewing machine.It is a 4.3v-6.I have never had any problems like this with this vehicle.The oil and filter are changed every 3000miles and this vehicle only has 40000 miles ont it.The oil pressure is good 40-50lbs.There is no bearing noise and the lifter noise goes away after you drive it about a mile or less.No indications of ny problems leading to this.the engine has always sounded and performed well.I dont know if its just a couple making noise or more.Icant see the lifters going bad overnight.Again oil pressure is good.Any ideas?