Mine had the same flat spot, especially noticable once I'd put a straight through exhaust on. Did seem to go once I'd had my chains / tensioners replaced at 160K miles. So my 2p worth is that Matt's diagnosis is valid. Vr6 boy might be right that the chains stretch, up to a point, almost immediatly in which case the engine is probably designed for it, but if they stretch too far, surely the top sprocket can become slightly out of sync with the drive cog, throwhing the timing out a smidge. With my new chains mine roars less as well, which I certainly attribute to the timing being better. TG