my windstar engine knocks at 98 k miles what could it be? every nowand again white smoke will come out of the exhaust pipe

What year??? Also white smoke not very good.. Maybe head gasket issue.. Does it smoke all the time or just when up to operation temp.?? Does it knock all the time or just under cold start.???

1995 3.8L winstar had issues with head gasket.
1996 and newer are good with head gaskets, but lower intake manifold gaskets are an issue.
Coolant into oil will cause damage to bearings.
You can have the oil tested at Blackstone Labs to see if there is coolant in the oil
http://www.blackstone-labs.com/

The test is easy, and not expensive.

You will NOT be able to "eyeball" the dipstick to find coolant in the oil
Damage will be happening LONG before you have that much coolant in the oil.

I believe many 96's suffered from head gasket issues too. Only the 95's had the offical recall.

My 96's headgasket started to leak with about 99 k mikes. they wanted 1500 to fix it. I priced a used engine with less miles on it for 2000 installed, so I thought I would go with that when it died. The van ran to 146k and I sold it running. It would blow a little white smoke when it started up, then after a minute, the head gasket would somehow make itself happy. The antifreeze consumption was very little and the vehicle was fine with antifreeze for 3k miles or so.

If your engine pings, you might have the 99-01 p171 issue without the codes. My 99 pinged a long time before throwing the codes.