Cold Start Engine Knock... Please Help!

[mkang310]

I have a '99 lexus gs300 with 120k miles on it and have always used 91 octane in it. Everytime I start the engine, especially when its cold there is a loud knocking sound which seems tom be coming from the lower block of the engine. As soon as the car is in motion or i give it some gas the sound is gone and the car runs perfectly. After reading other forms it seems like the symptoms might be pointing towards a rod knock, but im not sure. I use synthetic 10w30, tried using 10w 50 to fix the problem but it just got worse. Im hoping i dont have to replace the entire engine. Any ideas of what it might be?