Timer.Change takes a lock on a singleton instance (http://referencesource.microsoft.com/#mscorlib/system/threading/timer.cs)

03-TimerContention-02.cs

internal bool Change(uint dueTime, uint period)
{
    bool success;

    lock (TimerQueue.Instance) // Global lock.
    {
        if (m_canceled)
            throw new ObjectDisposedException(null, Environment.GetResourceString("ObjectDisposed_Generic"));

        // prevent ThreadAbort while updating state
        try { }
        finally
        {
            m_period = period;

            if (dueTime == Timeout.UnsignedInfinite)
            {
                TimerQueue.Instance.DeleteTimer(this);
                success = true;
            }
            else
            {
                if (FrameworkEventSource.IsInitialized && FrameworkEventSource.Log.IsEnabled(EventLevel.Informational, FrameworkEventSource.Keywords.ThreadTransfer))
                    FrameworkEventSource.Log.ThreadTransferSendObj(this, 1, string.Empty, true);

                success = TimerQueue.Instance.UpdateTimer(this, dueTime, period);
            }
        }
    }

    return success;
}